@article {KNZ002013, title = {Effects of vegetation rooting characteristics on carbonate weathering and critical zone evolution}, journal = {Earth and Planetary Science Letters}, year = {In Press}, keywords = {LTER-KNZ}, author = {Wen, H. and Sullivan, P.L. and G. L. Macpherson and Li, L.} } @article {KNZ002016, title = {Towards a new conceptual model for groundwater flow in merokarst systems: Insights from multiple geophysical approaches}, journal = {Hydrology and Earth System Science}, volume = {34}, year = {2020}, pages = {4697-4711}, keywords = {LTER-KNZ}, doi = {10.1002/hyp.13898}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/hyp.13898}, author = {Sullivan, P.L. and Zhang, C. and Behm, M. and Zhang, F. and G. L. Macpherson} } @article {KNZ001919, title = {Dust, impure calcite, and phytoliths: modeled alternative sources of chemical weathering solutes in shallow groundwater}, journal = {Chemical Geology}, volume = {527}, year = {2019}, pages = {118871}, abstract = {

In highly reactive, carbonate terrains that constitute more than one-fifth of critical zone landscapes, quantifying bedrock weathering processes may require understanding the realities of carbonate mineral impurity on solubility, biotically-produced minerals as an integral part of leaky biogeochemical cycles, and dust-deposited minerals as important high-surface-area, first-contact solute sources. The potential impact of these processes has not been thoroughly investigated as groundwater solutes are mostly thought to be sourced from chemical reactions with soil and bedrock, although dust is often viewed as an important delivery mechanism of nutrients to other ecosystems, including those in mountain and tropical settings. We present results of computer and hand-calculated (manual) inverse modeling of two years of stream-water chemistry, spanning a dry and a wet year, for a groundwater-fed headwater stream at the Konza Tallgrass Prairie Long-Term Ecological Research Site in northeastern Kansas. Weathering of the limestone and shale bedrock at the site provide a possible source of solutes to the groundwater-fed stream, but our modeling suggests an alternate source considering the pathway of groundwater recharge likely encounters highly reactive phases before encountering bedrock. We used the mineralogy and geochemistry of local dust collected previously, estimates of a possible chemical composition of phytolith containing potassium, and an impure calcite representing measured limestone bedrock chemistry to show that chemical reactions with the current dust flux are adequate to account for the groundwater chemistry. Average annual amounts were about: 1) 380 kg ha\−1 yr\−1 dust dissolved, 2) 80 kg ha\−1 yr\−1 bedrock carbonate dissolved, and 3) 320 kg ha\−1 yr\−1 phytoliths precipitated. Small amounts of cation exchange were also required to balance the models. There were only small differences between the computer and manual inverse models; both methods resulted in up to 4.6 times more mass of dust dissolved than bedrock. We suggest that dust weathering may be a process that occurs widely, considering the ubiquitous dust flux in continental regions.

}, keywords = {LTER-KNZ, Chemical weathering, Critical zone, Dust, Dust deposition, Headwater stream, Inverse model, Karst, Merokarst}, doi = {10.1016/j.chemgeo.2018.08.007}, url = {https://linkinghub.elsevier.com/retrieve/pii/S000925411830398X}, author = {G. L. Macpherson and Sullivan, P.L.} } @article {KNZ001962, title = {Evolution of carbonate and karst critical zones}, journal = {Chemical Geology}, volume = {527}, year = {2019}, pages = {119223}, abstract = {

Carbonate terrains (CT) underlie one-fifth of terrestrial, ice-free land and are an important supply of potable water to the world\&$\#$39;s population, and yet processes endemic to CT critical zones (CZ) and responses of these processes to climatic and anthropogenic pressures are not well understood. Given the rapid dissolution rates and ability to generate well-developed networks of secondary porosity these landscapes can be highly sensitive to impacts from climate change (e.g., modifications of temperature, precipitation, sea level) and human disturbance (e.g., water withdrawal/diversions, changes in land use/land cover). This special issue includes 16 papers focused on CT-CZ processes and potential responses to climatic and human perturbations. Five major themes emerge from these papers, namely: (1) anthropogenic climate and land use changes alter CT-CZ weathering rate and diagenesis, (2) metal and carbon fluxes in CT-CZ will respond to increasing hydrologic variance caused by climate change, (3) endogenous and exogenous processes operating over short time periods (\<10,000 yrs) form landscape patterns in carbonate terrains, (4) rates of carbonate mineral dissolution depend on vadose zone and soil thickness, and (5) open systems may not always promote greater carbonate weathering rates in CT-CZ. These findings reflect the importance of carbonate minerals in Earth\&$\#$39;s CZ, both in terms of processes unique to carbonate minerals, as well as a predictor of future responses to anthropogenic and environmental changes.

}, keywords = {LTER-KNZ}, doi = {10.1016/j.chemgeo.2019.06.023}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0009254119303183}, author = {Sullivan, P.L. and G. L. Macpherson and Martin, J.B. and Price, R.M.} } @article {KNZ001923, title = {Exploring methods of measuring CO2 degassing in headwater streams}, journal = {Sustainable Water Resources Management}, volume = {5}, year = {2019}, pages = {1765{\textendash}1779}, abstract = {

Carbon dioxide (CO2) degassed from ungauged, headwater streams has a significant role in carbon cycling and climate change, making the precise measurement of the degassing of critical importance. Although methods exist for quantifying degassing rates in large bodies of water (seawater, lakes), these methods are often considered invalid for measuring degassing rates in small, turbulent, groundwater fed headwater streams. This manuscript reviews the physics of gas transfer across the stream-atmosphere interface and provides an in-depth critical review of the available methods of measuring CO2 degassing. Further, it discusses applications for some of these methods in small headwater streams and other low-order streams that are dominated by discharged groundwater. Of the methods reviewed, almost all produce fairly low precision and do not compare well with other methods tested in the same location. We suggest much more work is needed to improve the precision and accuracy of field-measured gas transfer coefficients, both by applying multiple methods in the field and by controlled laboratory experiments.

}, keywords = {LTER-KNZ}, doi = {10.1007/s40899-019-00332-3}, url = {https://link.springer.com/article/10.1007/s40899-019-00332-3}, author = {Rawitch, M. J. and G. L. Macpherson and A.E. Brookfield} } @article {KNZ001920, title = {How landscape heterogeneity governs stream water concentration-discharge behavior in carbonate terrains (Konza Prairie, USA)}, journal = {Chemical Geology}, volume = {527}, year = {2019}, pages = {118989}, abstract = {

Mounting evidence suggests ecosystem changes that alter subsurface water fluxes and carbon dioxide concentrations in carbonate terrains may drive measurable changes in chemical weathering rates, stream water chemistry, and flow path evolution on human timescales. We test this idea by exploring if the encroachment of woody vegetation into grasslands in a carbonate terrain landscape at the Konza Prairie (KS, USA) has resulted in differences in landscape-stream connectivity and, thus, the behavior of stream water solutes. Woody encroachment (up to 60\% cover) at Konza has been observed on watersheds, particularly those that experience a fire return interval of four years or greater. We focus on three headwater catchments (two grassland and one woody-encroached) and a downstream confluence, and analyze stream water discharge and chemistry (major anions, cations, and dissolved nutrients) measured from 2015 to 2016.

We observe that the woody-encroached watershed exhibits a greater area-normalized solute flux and greater degree of chemodynamic behavior for most geogenic species compared to the less encroached grassland watersheds. The downstream confluence exhibits the most chemostatic behavior for these same solutes compared to the low order watersheds. We interpret the chemodynamic behavior of the woody-encroached watersheds to arise from a greater diversity of flow paths and solute sources that contribute to this stream. End member mixing analysis (EMMA) supports this hypothesis but also indicates a possible \“missing\” end member which we interpret to be solutes likely derived from clay weathering along limestone-mudstone boundaries. We invoke differences in rooting systems between grass and woody species to explain the differences in flow paths and solute generation between these headwater sites given that they sit adjacent to each other, dissect the same nearly horizontal (dip 0.1\–0.21\°NW) lithologic units, and experience the same climate. If these processes hold true at other sites, then the globally observed encroachment of woody vegetation into grasslands may deepen flow paths and enhance chemical weathering fluxes from ecosystems, and over long-time periods alter the trajectory of soil development and landscape evolution.

}, keywords = {LTER-KNZ, Critical zone, End member mixing analysis, Karst, woody encroachment}, doi = {10.1016/j.chemgeo.2018.12.002}, url = {https://doi.org/10.1016/j.chemgeo.2018.12.002}, author = {Sullivan, P.L. and Stops, M.W. and G. L. Macpherson and Li, L. and Hirmas, D.R. and W. K. Dodds} } @article {KNZ001971, title = {Increasing groundwater CO2 in a midcontinent tallgrass prairie: Controlling factors}, journal = {E3S Web of Conferences}, volume = {98}, year = {2019}, pages = {06008}, abstract = {

Alkalinity and groundwater CO2 have increased linearly from 1991\–2017 at the Konza Prairie Biological Station (KPBS), a tallgrass prairie research site in northeastern Kansas. The projected increase in groundwater alkalinity (as HCO3-) and CO2 based on an earlier trend was confirmed in 2016, with predictions nearly equal to recent values (e.g., 408 ppm vs 410 ppm as HCO3-, respectively). Both the water balance and groundwater CO2 trends within the study watershed could be impacted by long-term changes in land use and climate: 1) encroachment of woody vegetation (1983\–2012) as a result of the 4-year fire return interval, 2) re-introduction of bison (phased in, 1994\–2006), 3) increases in air temperature, and 4) changes in precipitation patterns. If only linear processes are driving the observed water chemistry changes, then the linear increase in air temperature (1983\–2017) that stimulates soil respiration may be the most likely factor enhancing groundwater HCO3- and CO2, as air temperature has risen ~1 to 1.4\°C over 34 years. If groundwater chemistry is driven by more threshold behaviour, woody encroachment, which was linear but in three distinct phases, may drive groundwater chemistry. The ~2 to 3\‰ decrease in the discontinuous δ13C data in the groundwater-dominated stream suggests enhanced inputs of microbially-respired labile carbon, CO2 sourced from C3 (woody vegetation), or a combination of the two.

}, keywords = {LTER-KNZ}, doi = {10.1051/e3sconf/20199806008}, url = {https://www.e3s-conferences.org/10.1051/e3sconf/20199806008}, author = {G. L. Macpherson and Sullivan, P.L. and Stotler, R.L. and Norwood, B.S.}, editor = {Chudaev, O. and Kharaka, Y. and Harmon, R. and Millot, R. and Shouakar-Stash, O.} } @article {KNZ001921, title = {Watershed-scale chemical weathering in a merokarst terrain, northeastern Kansas, USA}, journal = {Chemical Geology}, volume = {527}, year = {2019}, pages = {118988}, abstract = {

Carbonate weathering in the merokarst landscape at the Konza Prairie (near Manhattan, KS) is governed by kinetically rapid reaction rates, similar to other carbonate settings. The geology of the site consists of repeating couplets of shale and limestone mantled by Pleistocene loess, on which most soils are built. The headwater stream that is the subject of this study is intermittent and well connected to the secondary porosity in the limestone aquifers, such that the water chemistry is groundwater-dominated. Despite increasing in situ weathering rates shown by increasing solute concentrations, long-term (~21 of 24 years) decline in stream-water discharge has reduced chemical denudation. The current chemical denudation rate is 0.02 mm yr\−1. Limestone bedrock thickness (~26.6 m) is 38\% of total bedrock thickness (~70 m); the present landscape would have taken 1\–4 million years to dissolve at current weathering rates. Despite paleogeographic reconstructions suggesting exposure for the past 260 Ma, estimates of the time to erode younger strata using current physical and chemical denudation rates are 80\–159 Ma, still much less than the age of the rocks. An alternate source of material, dust, is proposed that may slow limestone weathering, which in turn protects shales from physical erosion, thereby slowing landscape evolution.

}, keywords = {LTER-KNZ}, doi = {10.1016/j.chemgeo.2018.12.001}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0009254118305874?via\%3Dihub}, author = {G. L. Macpherson and Sullivan, P.L.} } @article {KNZ001821, title = {Developing a conceptual framework of landscape and hydrology on tallgrass prairie: A critical zone approach}, journal = {Vadose Zone Journal}, volume = {17}, year = {2018}, pages = {1 - 11}, abstract = {

Agricultural intensification and urbanization have greatly reduced the extent of tallgrass prairie across North America. To evaluate the impact of these changes, a reference ecosystem of unperturbed prairie is required. The Konza Prairie Biological Station in northeastern Kansas is a long-term research site at which a critical zone approach has been implemented. Integration of climatic, ecologic, and hydropedologic research to facilitate a comprehensive understanding of the complex environment provides the basis for predicting future aquifer and landscape evolution. We present a conceptual framework of the hydrology underpinning the area that integrates the extensive current and past research and provides a synthesis of the literature to date. The key factors in the hydrologic behavior of Konza Prairie are climate, ecology, vadose zone characteristics and management, and groundwater and bedrock. Significant interactions among these factors include bedrock dissolution driven by cool-season precipitation and hence a climatic control on the rate of karstification. Soil moisture dynamics are influenced at various timescales due to the short- and long-term effects of prescribed burning on vegetation and on soil physical characteristics. The frequency of burning regimes strongly influences the expansion of woody species in competition with native tallgrasses, with consequent effects on C and N dynamics within the vadose zone. Knowledge gaps exist pertaining to the future of Konza Prairie (a model for US tallgrass prairie)\—whether continued karstification will lead to increasingly flashy and dynamic hydrology and whether compositional changes in the vegetation will affect long-term changes in water balances.

}, keywords = {LTER-KNZ}, doi = {10.2136/vzj2017.03.0069}, url = {https://dl.sciencesocieties.org/publications/vzj/pdfs/17/1/170069}, author = {Vero, S. E. and G. L. Macpherson and Sullivan, P.L. and A.E. Brookfield and Jesse B. Nippert and Kirk, M. F. and Datta, S. and Kempton, P.} } @article {KNZ001932, title = {Large and active CO 2 uptake by coupled carbonate weathering}, journal = {Earth-Science Reviews}, volume = {182}, year = {2018}, pages = {42 - 49}, abstract = {

Carbonate mineral weathering coupled with aquatic photosynthesis on the continents, herein termed coupled carbonate weathering (CCW), represents a current atmospheric CO2 sink of about 0.5 Pg C/a. Because silicate mineral weathering has been considered the primary geological CO2 sink, CCW\&$\#$39;s role in the present carbon cycle has been neglected. However, CCW may be helping to offset anthropogenic atmospheric CO2 increases as carbonate minerals weather more rapidly than silicates. Here we provide an overview of atmospheric CO2 uptake by CCW and its impact on global carbon cycling. This overview shows that CCW is linked to climate and land-use change through changes in the water cycle and water-born carbon fluxes. Projections of future changes in carbon cycling should therefore include CCW as linked to the global water cycle and land-use change.

}, keywords = {LTER-KNZ}, doi = {10.1016/j.earscirev.2018.05.007}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0012825217306153}, author = {Liu, Zaihua. and G. L. Macpherson and Groves, Chris. and Martin, J.B. and Yuan, Daoxian. and Zeng, Sibo.} } @article {KNZ001779, title = {Effects of changing meteoric precipitation patterns on groundwater temperature in karst environments}, journal = {Groundwater}, volume = {55}, year = {2017}, pages = {227-236}, abstract = {

Climate predictions indicate that precipitation patterns will change and average air temperatures will increase across much of the planet. These changes will alter surface water and groundwater temperatures which can significantly affect the local and regional environment. Here, we examine the role of precipitation timing in changes to groundwater temperature in carbonate-karst aquifers using measured groundwater level and temperature data from the Konza Prairie Long-Term Ecological Research Site, Kansas. We demonstrate that shifts to increased cool-season precipitation may mitigate the increases in groundwater temperature produced by increases in average annual air temperature. In karst, the solution-enlarged conduits allow faster and focused recharge, and the recharge-event temperature can strongly influence the groundwater temperature in the aquifer. Our field data and analysis show that predictions of future groundwater conditions in karst aquifers need to consider changes in precipitation patterns, in addition to changes to average annual air temperature.

}, keywords = {LTER-KNZ}, doi = {10.1111/gwat.12456}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gwat.12456}, author = {A.E. Brookfield and G. L. Macpherson and Covington, M.} } @mastersthesis {KNZ001778, title = {Stream CO2 degassing: review of methods and laboratory validation of floating chambers}, volume = {MS Thesis}, year = {2016}, school = {University of Kansas}, type = {M.S. Thesis}, address = {Lawrence, Kansas}, keywords = {LTER-KNZ}, url = {https://kuscholarworks.ku.edu/bitstream/handle/1808/21889}, author = {Rawitch, M. J.} } @mastersthesis {KNZ001659, title = {Inorganic and organic carbon variations in surface water, Konza Prairie LTER Site, USA, and Maolan Karst Experimental Site, China}, volume = {MS Thesis}, year = {2014}, school = {University of Kansas}, type = {M.S. Thesis}, address = {Lawrence, KS}, abstract = {

Two natural groundwater-fed streams were selected to examine the diurnal trends of water geochemistry, CO2 emissions, and sources of OC on 25 May 2012 at the Konza Prairie Long-Term Ecological Research (LTER) site, USA and 29 August - 30 August 2012 at the Maolan Karst Experimental (Maolan) Site, China. For the stream at the Konza LTER site, little variation in water chemistry was observed among the upstream, midstream and downstream locations, indicating the groundwater and stream water chemistry was mostly stable on a daily basis. 13CDIC was the highest at the downstream site due to the largest CO2 degassing. The autochthonous particulate organic carbon (POC) fraction at the upstream, midstream and downstream sites was 12-35\%, 39-65\% and 75-88\%, respectively. Estimation of the C/N ratio for POC samples at the three locations was 10.4-15.0, 14.0-15.9 and 12.2-13.3. This is comparable to previously measured C/N ratios of suspended POC. For the stream at the Maolan site, there was little or no diel variation in the spring water physical and chemical parameters. However, all parameters show distinct diel changes in the spring-fed midstream pond with flourishing submerged plants. Temperature, pH, DO, SIC, 13CDIC increased during the day and decreased at night, while EC, [HCO3-], [Ca2+], and pCO2 behaved in the opposite sense. Strong aquatic photosynthesis was indicated from maximum DO values (two to three times higher than normal water equilibrated with atmospheric O2). In the downstream pond with fewer submerged plants but larger volume, all parameters displayed similar trends to the midstream pond but with much less change. We attribute this pattern to the lower biomass/water volume ratio. The diel variations in the two ponds resulted from the aquatic photosynthetic effect, demonstrating that natural surface water systems may constitute an important sink of carbon.

}, keywords = {LTER-KNZ}, url = {http://hdl.handle.net/1808/14546}, author = {Liu, Huan.} } @article {KNZ001513, title = {The effect of precipitation events on inorganic carbon in soil and shallow groundwater, Konza Prairie LTER Site, NE Kansas, USA}, journal = {Applied Geochemistry}, volume = {27}, year = {2012}, pages = {2356 -2369}, abstract = {

Monthly sampling for 1 year at the Konza Prairie LTER (Long-Term Ecological Research) Site in northeastern Kansas shows a connection between the annual cycles of CO2 in soil air and shallow groundwater DIC (dissolved inorganic C). Soil air CO2 reached 6\–7\% in July to mid-August, when moisture was not limiting to soil respiration. Following the annual maximum there was a sequential decrease in CO2 in three soil horizons to less than 0.5\% because of moisture deficiency in the late summer and temperature decline in the fall and winter. Groundwater pCO2 reached its maximum of 5\% in October; the lag-time of 2\–3 months may correspond to the travel time of soil-generated CO2 to the water table. The time-variable CO2 caused an annual carbonate-mineral saturation cycle, intensifying limestone dissolution and DIC production when CO2 was high. The C flux depended on respiration and rainfall regimes, and had two main pathways. Transport of soil CO2 in the dissolved form with diffuse flow of recharge water was the most effective during the growing season so long as soil moisture was present. Downward movement of gaseous CO2 and equilibration with groundwater at the water table was favorable in July to August. Storm rainfall events recharged the aquifer within a few hours through preferential flow and stream\–groundwater interaction, resulting in dilution of groundwater rather than forcing entrapped CO2 downward. Calculated C flux from the unsaturated zone to the unconfined aquifer in the monitoring period was 0.26 \± 0.03 M/m2/a of C, which is less than 1\% of the CO2 that is released by soil to the atmosphere via efflux. However, meteoric precipitation was only 72\% of average annual precipitation during the study period, so this study represents dry-condition flux. In addition, increased respiration rates due to warming of the atmosphere have the potential to cause a higher C flux to the saturated zone, intensifying weathering and groundwater acidification, so that further study is suggested.

}, keywords = {LTER-KNZ}, doi = {10.1016/j.apgeochem.2012.07.008}, url = {https://www.sciencedirect.com/science/article/pii/S0883292712001734?via\%3Dihub}, author = {Tsypin, M. and G. L. Macpherson} } @mastersthesis {KNZ001487, title = {Dissolved inorganic carbon in soil and shallow groundwater, Konza Prairie LTER Site, NE Kansas, USA}, volume = {MS Thesis}, year = {2011}, school = {University of Kansas}, type = {M.S. Thesis}, address = {Lawrence, KS}, abstract = {

Sources and seasonal trends of dissolved inorganic carbon (DIC) in a shallow limestone aquifer were studied for 1 year at the Konza Prairie LTER (Long-Term Ecological Research) Site in northeastern Kansas, from spring 2010 to spring 2011. Annual cycles of soil air CO2, groundwater DIC, and isotope characteristics showed a strong dependency on weather conditions and soil respiration. Soil air CO2 reached its annual maximum in the middle of the growing season, when moisture was not limiting to soil respiration. Following the maximum, the CO2 decreased because of moisture deficiency in the late summer and temperature decline in the fall and winter. The decrease began first in the shallowest part of the soil and last in the deepest part. Groundwater CO2 reached its annual maximum in October; this lag-time between the soil and groundwater CO2 maxima of 2-3 months may correspond to the travel time of soil-generated CO2 to the water table. The time-variable CO2 caused an annual carbonate-mineral saturation cycle, intensifying limestone dissolution, thus soil CO2 and carbonate minerals are the two main sources of DIC in soil and groundwater. The stable carbon isotope composition of soil air CO2 and DIC exhibited primarily C4 plant signature and were similar to that of soil organic matter, suggesting that both root and bacterial respiration are sources of CO2. DIC was enriched in 7-10 per mil PDB relative to the CO2 source due to isotope fractionation in a system open to soil CO2; the enrichment was smallest under highest pCO2. For this reason, d13CDIC was out of phase with DIC, the lightest in the late growing season. The carbon flux from the unsaturated zone to the unconfined aquifer during the year was variable depending on respiration and precipitation regimes, and had two main pathways. Transport of soil CO2 in the dissolved form with diffuse flow of recharge water was the most effective during the entire growing season. Downward movement of gaseous CO2 and equilibration with groundwater at the water table was possible in July to August. Storm rainfall events rapidly recharged the aquifer through preferential flow and stream-groundwater interaction. Rather than forcing soil gases downward because of water-saturated pores, the main effect of these events was dilution of groundwater. The calculated flux was about 0.3 M/m2/yr of C, which is less than 1\% of the CO2 that is released by soil to the atmosphere via efflux. However, the climate prediction of increased respiration rates, temperature, and frequency of extreme rainfall events has the potential to cause higher carbon flux to the saturated zone, intensifying weathering and groundwater acidification.

}, keywords = {LTER-KNZ}, url = {http://hdl.handle.net/1808/10386}, author = {Tsypin, Mikhail} } @article {KNZ001427, title = {From precipitation to groundwater baseflow in a native prairie ecosystem: a regional study of the Konza LTER in the Flint Hills of Kansas, USA}, journal = {Hydrology and Earth System Sciences}, volume = {15}, year = {2011}, pages = {3181 -3194}, abstract = {

Methods are developed to study hydrologic interactions across the surficial/groundwater interface in a native prairie ecosystem. Surficial ecohydrologic processes are simulated with the USDA\&$\#$39;s EPIC model using daily climate data from the Kansas Weather Data Library, vegetation and soil data from the USDA, and current land-use management practices. Results show that mean annual precipitation (from 1985\–2005) is partitioned into 13\% runoff regionally and 14\% locally over the Konza LTER, lateral flow through soil is 1\% regionally and 2\% locally, groundwater recharge is 11\% regionally and 9\% locally, and evapotranspiration accounts for the remaining 75\%. The spatial distribution of recharge was used in a regional Modflow groundwater model that was calibrated to existing groundwater observations and field measurements gathered for this study, giving a hydraulic conductivity in the Flint Hills region of 1\–2 m day\−1 with a local zone (identified here) of 0.05\–0.1 m day\−1. The resistance was set to fixed representative values during model calibration of hydraulic conductivity, and simple log-log relations correlate the enhanced recharge beneath ephemeral upland streams and baseflow in perennial lowland streams to the unknown resistance of the streambeds. Enhanced recharge due to stream transmission loss (the difference between terrestrial runoff and streamflow) represents a small fraction of streamflow in the ephemeral upland and the resistance of this streambed is 100 000 day. Long-term baseflow in the local Kings Creek watershed (2\% of the groundwater recharge over the watershed) is met when the resistance of the lowland streambed is 1000 day. The coupled framework developed here to study surficial ecohydrological processes using EPIC and groundwater hydrogeological processes using Modflow provides a baseline hydrologic assessment and a computational platform for future investigations to examine the impacts of climate change, vegetative cover, soils, and management practices on hydrologic forcings.

}, keywords = {LTER-KNZ}, doi = {10.5194/hess-15-3181-2011}, url = {https://www.hydrol-earth-syst-sci.net/15/3181/2011/}, author = {Steward, D.R. and Yang, X. and Lauwo, S.Y. and Staggenborg, S.A. and G. L. Macpherson and Welch, S.M.} } @article {KNZ001268, title = {CO2 distribution in groundwater and the impact of groundwater extraction on the global C cycle}, journal = {Chemical Geology}, volume = {264}, year = {2009}, pages = {328 -336}, abstract = {

Growing population and wealth, agricultural expansion, changing land use, and mechanized irrigation increase the demand on water resources. Groundwater accounts for about 20\% of global water use, with agriculture the largest consumer. Groundwater CO2 partial pressures are typically \∼ 10\–100 times higher than atmospheric, so CO2 degassing is an unavoidable consequence of groundwater extraction. For regional studies, very good estimates of groundwater CO2 in clastic aquifers can be calculated for temperate-climate regions from: log10(pCO2)=log10(HCO\−3)\−pH+7.749 where the constant is the sum of the negative logarithm (base 10) of the Henry\&$\#$39;s Law constant for CO2 and first association constant for H2CO3 at 13 \°C, and titration alkalinity can be substituted for activity of HCO3\−. Groundwater CO2 is not uniformly distributed in groundwater, being most variable near the water table, lowest at intermediate depth, and highest in deep aquifers where non-potable water is typical. Geochemical speciation modeling is required to assess CO2 released when groundwater equilibrates with the atmosphere. A suite of potable water types were modeled for this purpose, with the result that current groundwater production is estimated to release \∼ 0.01 to 0.03 Pg C year\− 1, similar to CO2 from volcanic emissions and \∼ 200 times less than other human-sourced CO2 emissions. Determination of dissolved CO2 depends on accuracy of pH measurements: CO2 released due to groundwater use may be underestimated if reported pH measurements do not represent in situ pH.

}, keywords = {LTER-KNZ, Carbon dioxide, CO2, Global carbon cycle, groundwater, Groundwater extraction}, doi = {10.1016/j.chemgeo.2009.03.018}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0009254109001399?via\%3Dihub}, author = {G. L. Macpherson} } @proceedings {KNZ001267, title = {Groundwater CO2: Is it responding to atmospheric CO2?}, volume = {II}, year = {2009}, pages = {995 -998}, keywords = {LTER-KNZ}, author = {G. L. Macpherson} } @article {KNZ001230, title = {Increasing shallow groundwater CO2 and limestone weathering, Konza Prairie, USA}, journal = {Geochimica et Cosmochimica Acta}, volume = {72}, year = {2008}, pages = {5581 -5599}, abstract = {

In a mid-continental North American grassland, solute concentrations in shallow, limestone-hosted groundwater and adjacent surface water cycle annually and have increased steadily over the 15-year study period, 1991\–2005, inclusive. Modeled groundwater CO2, verified by measurements of recent samples, increased from 10\−2.05 atm to 10\−1.94 atm, about a 20\% increase, from 1991 to 2005. The measured groundwater alkalinity and alkaline-earth element concentrations also increased over that time period. We propose that carbonate minerals dissolve in response to lowered pH that occurs during an annual carbonate-mineral saturation cycle. The cycle starts with low saturation during late summer and autumn when dissolved CO2 is high. As dissolved CO2 decreases in the spring and early summer, carbonates become oversaturated, but oversaturation does not exceed the threshold for precipitation. We propose that groundwater is a CO2 sink through weathering of limestone: soil-generated CO2 is transformed to alkalinity through dissolution of calcite or dolomite. The annual cycle and long-term increase in shallow groundwater CO2 is similar to, but greater than, atmospheric CO2.

}, keywords = {LTER-KNZ}, doi = {10.1016/j.gca.2008.09.004}, url = {https://www.sciencedirect.com/science/article/pii/S001670370800536X?via\%3Dihub}, author = {G. L. Macpherson and Roberts, J.A. and John M. Blair and Townsend, M.A. and Fowle, D.A. and Beisner, K.R.} } @article {KNZ001099, title = {Carbon isotope variation in modern soils of the tallgrass prairie:Analogues for the interpretation of isotopicrecords derived from paleosols}, journal = {Quaternary International}, volume = {162-163}, year = {2007}, pages = {3 -20}, abstract = {Use of stable carbon isotope data from paleosols to reconstruct past plant community structure (C3 vs. C4) has become commonplace. In an effort to improve our ability to make isotope-based reconstructions and to better appreciate the pitfalls, investigations were conducted on both modern soils and paleosols in the Kansas grasslands. Stable carbon isotope data were derived from soils and vegetation on the near-pristine, C4-dominated grassland of the Konza Tallgrass Prairie Long-Term Ecological Research (LTER) site in northeastern Kansas. In order to evaluate variation of δ13C within the landscape, two levels of sampling were employed: 2 m-deep upland cores extracted to assess variation with depth in the soil profile, and, to assess variability across the landscape, surface samples along two transects and from within a 660{\texttimes}690 m grid. For transect and grid points, both the upper 2 cm of sediment and the aboveground biomass were collected. Core samples taken at Konza reveal that soil organic carbon was depleted in 13C within the upper 10{\textendash}20 cm relative to the remainder of the soil solum below, a phenomenon previously reported. In transects and the sample grid, soil organic carbon from soil surfaces was consistently more depleted in 13C than aboveground tissue of associated vegetation samples. Slope, azimuth, and insolation were computed from field data and a high-resolution DEM of the sample grid, but these variables offered no significant explanation of the spatial variability in δ13C data from soil organic carbon. The observation that modern landscape position has little effect on δ13C over short distances, at least in a C4-dominated community, lends support to the application of δ13C analysis to buried soils for paleoenvironmental reconstruction where paleolandscape position is often unknown. Stable carbon isotope data were derived from paleosols at from the Beisel-Steinle site located to the west of Konza in central Kansas. Three soils were examined: the interstadial paleosol within the Gilman Canyon Formation (c. 38{\textendash}27k cal yr BP), the Late Pleistocene{\textendash}Holocene Brady Soil, and the modern surface soil. With the exception of isotopic depletion at the top of the surface soil, all three soils exhibited similar δ13C values of about14{\textperthousand}, the same value realized in the soil cores from Konza, suggesting close similarity among the plant communities of the interstade, the Late Pleistocene{\textendash}Early Holocene transition, and pristine prairie of today. This investigation emphasizes the need for additional research into the variation in stable carbon isotope signals vertically within the profile and across the landscape. Issues include the source of the near-surface depletion zone, the relationship between isotopic values in the soil versus those of the vegetation, and variation in soil δ13C values δ13C throughout the landscape.}, keywords = {LTER-KNZ}, doi = {10.1016/j.quaint.2006.10.036}, author = {Johnson, W.C. and Willey, K.L. and G. L. Macpherson} } @article {KNZ001050, title = {Sources of Sr and implications for weathering of limestone under tallgrass prairie, northeastern Kansas}, journal = {Applied Geochemistry}, volume = {20}, year = {2005}, pages = {2325 -2342}, abstract = {Grasslands of north-central Kansas are underlain by carbonate aquifers and shale aquitards. Chemical weathering rates in carbonates are poorly known, and, because large areas are underlain by these rocks, solute fluxes are important to estimating global weathering rates. Grasslands exist where the amount of precipitation is extremely variable, both within and between years, so studies in grasslands must account for changes in weathering that accompany changes in precipitation. This study: (1) identifies phases that dominate chemical fluxes at Konza Prairie Biological Station (KPBS) and Long-Term Ecological Research Site, and (2) addresses the impact of variable precipitation on mineral weathering. The study site is a remnant tallgrass prairie in the central USA, representing baseline weathering in a mid-temperate climate grassland. Groundwater chemistry and hydrology in the 1.2 km2 watershed used for this study suggest close connections between groundwater and surface water. Water levels fluctuate seasonally. High water levels coincide with periods of precipitation plus low evapotranspiration rather than during precipitation peaks during the growing season. Precipitation is concentrated before recharging aquifers, suggesting an as yet unquantified residence time in the thin soils. Groundwater and surface water are oversaturated with respect to calcite within limitations of available data. Water is more dilute in more permeable aquifers, and water from one aquifer (Morrill) is indistinguishable from surface water. Cations other than Ca co-vary with each other, especially Sr and Mg. Potassium and Si co-vary in all aquifers and surface water, and increases in concentrations of these elements are the best indicators of silicate weathering at this study site. Silicate-weathering indices correlate inversely to aquifer hydraulic conductivity. 87Sr/86Sr in water ranges from 0.70838 to 0.70901, and it decreases with increasing Sr concentration and with increasing silicate-weathering index. Carbonate extracted from aquifer materials, shales, soil, and tufa has Sr ranging from about 240 (soil) to 880 ppm (Paleozoic limestone). 87Sr/86Sr ranges from 0.70834 {\textpm} 0.00006 (limestone) to 0.70904 {\textpm} 0.00019 (soil). In all cases, 87Sr/86Sr of aquifer limestone is lower than 87Sr/86Sr of groundwater, indicating a phase in addition to aquifer carbonate is contributing solutes to water. Aquifer recharge controls weathering: during periods of reduced recharge, increased residence time increases the total amount of all phases dissolved. Mixing analysis using 87Sr/86Sr shows that two end members are sufficient to explain sources of dissolved Sr. It is proposed that the less radiogenic end member is a solution derived from dissolving aquifer material; longer residence time increases its contribution. The more radiogenic end member solution probably results from reaction with soil carbonate or eolian dust. This solution dominates solute flux in all but the least permeable aquifer and demonstrates the importance that land-surface and soil-zone reactions have on groundwater chemistry in a carbonate terrain.}, keywords = {LTER-KNZ}, doi = {10.1016/j.apgeochem.2005.08.002}, author = {Wood, H.K. and G. L. Macpherson} } @article {KNZ00903, title = {Fast ground-water mixing and basal recharge in an unconfined, alluvial aquifer, Konza LTER Site, Northeastern Kansas}, journal = {Journal of Hydrology}, volume = {286}, year = {2004}, pages = {271 -299}, abstract = {Ground-water chemistry and water levels at three levels in a well nest were monitored biweekly for two and a half years in a shallow unconfined floodplain aquifer in order to study the dynamics of such shallow aquifers. The aquifer, in northeastern Kansas, consists of high porosity, low hydraulic conductivity fine-grained sediments dominated by silt and bounded by fractured limestone and shale bedrock. Results show that the aquifer underwent chemical stratification followed by homogenization three times during the study period. The length of time between maximum stratification and complete homogenization was 3{\textendash}5 months. The chemical parameters most useful for demonstrating the mixing trends were dissolved nitrate and sulfate. Higher nitrate concentrations were typical of unsaturated zone water and were sourced from fertilizer applied to the cultivated fields on the floodplain. Variations in sulfate concentrations are attributed to dissolution of rare gypsum in limestone bedrock and variable evapoconcentration in the unsaturated zone. The mixing of three chemically different waters (entrained, unsaturated-zone water; water entering the base of the floodplain aquifer; and water in residence before each mixing event) was simulated. The resident water component for each mixing event was a fixed composition based on measured water chemistry in the intermediate part of the aquifer. The entrained water composition was calculated using a measured composition of the shallow part of the aquifer and measurements of soil-water content in the unsaturated zone. The incoming basal water composition and the fractions of each mixing component were fitted to match the measured chemistry at the three levels in the aquifer. A conceptual model for this site explains: (1) rapid water-level rises, (2) water-chemistry changes at all levels in the aquifer coincident with the water-level rises, (3) low measured hydraulic conductivity of the valley fill and apparent lack of preferential flow pathways, (4) minuscule amounts of unsaturated-zone recharge, and (5) dissolved oxygen peaks in the saturated zone lagging water-level peaks. We postulate that rainfall enters fractures in bedrock adjacent to the floodplain. This recharge water moves rapidly through the fractured bedrock into the base of the floodplain aquifer. The recharge event through the bedrock causes a rapid rise in water level in the floodplain aquifer, and the chemistry of the deepest water in the floodplain aquifer changes at that time. The rising water also entrains slow-moving, nitrate-rich, unsaturated-zone water, altering the chemistry of water in the shallow part of the aquifer. Vertical chemical stratification in the aquifer is thus created by the change in water chemistry in the upper and lower parts of the saturated zone. As the water level begins to decline, the aquifer undergoes mixing that eventually results in homogeneous water chemistry. The rise in water level from the recharge event also displaces gas from the unsaturated zone that is then replaced as the water level declines following the recharge event. This new, oxygen-rich vadose-zone air equilibrates rapidly with saturated-zone water, resulting in a dissolved oxygen pulse in the ground water that peaks one-half to 2 months after the water-level peak. This oxygen pulse subsequently declines over a period of 2{\textendash}6 months.}, keywords = {LTER-KNZ, Chemical stratification, Dissolved oxygen, Floodplain aquifer, Ground-water recharge, Loess; Soil-water modeling}, doi = {10.1016/j.jhydrol.2003.09.016}, author = {G. L. Macpherson and Sophocleous, M.} } @proceedings {KNZ00948, title = {Field vs. lab pH in remote wells, Konza Prairie LTER site, and implications for limestone weathering}, year = {2004}, pages = {845 -848}, publisher = {A. A. Balkema Publishers, New York}, address = {Saratoga Springs, New York}, keywords = {LTER-KNZ}, author = {G. L. Macpherson} } @inbook {KNZ00642, title = {Hydrology and aquatic chemistry}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {159 -176}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Gray, L.J. and G. L. Macpherson and Koelliker, J.K. and W. K. Dodds}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @proceedings {KNZ00655, title = {Nitrate loading of shallow ground water, prairie vs. cultivated land, northeastern Kansas, USA}, year = {1998}, pages = {165 -168}, publisher = {A.A. Balkema}, address = {Roterdam}, keywords = {LTER-KNZ}, author = {G. L. Macpherson} } @article {KNZ00598, title = {Trace elements in recent algal carbonate at the Konza LTER site, northeastern Kansas, determination by laser ablation microprobe (LAM) ICPMS}, journal = {Geological Society of America}, volume = {29}, year = {1997}, pages = {1 -175}, keywords = {LTER-KNZ}, author = {G. L. Macpherson and Jackson, S.} } @article {KNZ00559, title = {Hydrogeology of thin-bedded limestones: the Konza Prairie Long-Term Ecological Research site, Northeastern Kansas}, journal = {Journal of Hydrology}, volume = {186}, year = {1996}, pages = {191 -228}, keywords = {LTER-KNZ}, author = {G. L. Macpherson} } @article {KNZ00460, title = {Source(s), fate and residence time of nitrate at two sites in Kansas--a comparison of carbonate and alluvial aquifers, Kansas Water Resources Research Institute Report, Contribution No. 312}, year = {1994}, pages = {81 -}, keywords = {LTER-KNZ}, author = {G. L. Macpherson and Schulmeister, M.K.} } @article {KNZ00414, title = {Preliminary assessment of nitrate at two sites in Kansas-comparison of alluvial aquifer and fractured limestone, Kansas Water Resources Research Institute Report, Contribution No. 305}, year = {1993}, pages = {39 -}, keywords = {LTER-KNZ}, author = {G. L. Macpherson} } @proceedings {KNZ00369, title = {Ground-water chemistry under tallgrass prairie, Central Kansas, USA}, year = {1992}, pages = {809 -812}, keywords = {LTER-KNZ, tallgrass prairie}, author = {G. L. Macpherson}, editor = {Kharaka, Y.K. and Maest, A.S.} }