Location of Sampling Stations

Samples are regularly collected from wells in watersheds N01B and N04D and from surface water locations in watersheds N01B, N04D, and N02B at Konza Prairie Biological Station. Specific information about each watershed’s sampling locations follows below.

Watershed N04d wells: Thirty-five wells are located in the lower third of the N04d watershed. In 1988 and 1990, the U.S.Geological Survey, Lawrence, KS, drilled 31 wells at 20 sites within the N04D watershed at the Konza Prairie Research Natural Area. In 1997, the Konza LTER program funded drilling of an additional 4 wells at 3 additional sites. The older wells range in depth from about 2 to 13 meters; the newer wells are about 12 m, 21 m, 27 m and 37 m deep. Wells are completed in discontinuous alluvium/colluvium or in bedrock. Some of the bedrock-aquifer wells are nested to include two to three limestones in the Permian-aged bedrock. Thin (1-2 meter) limestones alternating with thicker (2-4 meter) shales in the region; limestones are aquifers. Most bedrock wells sample the Morrill Limestone Member of the Beattie Limestone (stratigraphically lowest), two wells sample an unnamed, discontinuous limestone with the Stearns Shale, and several wells access one or two levels in the Eiss Limestone Member of the Bader Limestone. A single well, not presently used, samples the Middleburg Limestone Member of the Bader Limestone (stratigraphically highest). The wells sites or nests comprise four transects running approximately east-west across the watershed drainage, and occupy the lower one-quarter of the watershed surface area.

Watershed N01b wells: Five wells are located in the lower portion of the N01b watershed. All of the N01b wells were drilled in November of 2020 by the Kansas Geological Survey, Lawrence, KS. The wells are located at three sites along a single east-west transect perpendicular and to the west of the watershed drainage. They range in depth from about 13 to 28 m, with two of the wells screened in the Eiss limestone and three screened in the Morrill Limestone. The two Eiss Limestone wells are adjacent to two Morrill Limestone wells at mid and upper hillslope positions. The third Morrill Limestone well is located at a lower position adjacent to riparian woody vegetation. No Eiss Limestone well was installed there because the surface elevation is below the horizon of the Eiss Limestone. The well transect is located near the watershed outlet.

Surface water sampling locations: Stream samples are collected near the outlet of watersheds N01B, N04D, and N02B (1-1 stream location) when the stream is flowing continuously, which tends to occur in spring and early summer. Additionally, two upgradient stream locations (3-1 stream, 4-1 stream) are sampled in watershed N04D. Those locations coincide with a pool of stream water that exists year-round and is fed by groundwater discharge from the Upper Eiss Limestone.

Frequency of Sampling

Water samples and water level measurements have been collected regularly since 1990. However, the procedures and variables collected have evolved over time. The sections below describe approaches during specific time intervals.

1990 through 2019: Water samples are currently collected every 4-6 weeks, as weather permits, from up to 7 wells and 2 stream locations; depth-to-water is measured in all wells (1994-present). In 1990, samples were collected quarterly from all wells and all 4 stream locations that were not dry. From 1991-1993, water samples were collected from all wells and all 4 stream locations that were not dry. Variable Measured: 1) Depth to water (water level) in wells.2) Dissolved Na, K, Li, Ca, Mg, Sr, Ba, SO4, Cl, F, NO3-N, HPO4-P, total alkalinity, Si, and B in all well or stream samples with sufficient water. 3) Field pH in some well and stream samples.

Field Methods: Water level measurement. Depth to water is measured after removing the PVC well cap and allowing the well to 'breathe' for several minutes, using a water level meter (Solinist 101, with flat polyethylene cable and stainless steel probe). Reference points (measuring points) are marked on the well casings. Reproducibility of measurements is on the order of 6 mm (0.02 feet). Prior to April 2015, well depth was also measured. Water chemistry: Field personnel wear powder-free latex or other plastic gloves during all procedures. Wells are bailed until approximately two well-casing-plus-annular space volumes of water have been removed. Samples for chemical analysis are carefully bailed from the wells using a one-liter Teflon® bailer suspended on Teflon®-coated steel wire. Samples are emptied into dedicated, pre-cleaned (with distilled water in the lab), dried, and pre-rinsed (with groundwater in the field), 2-liter, low-density polyethylene (LDPE) jugs using a Teflon® bottom- emptying device inserted into the bailer after it is removed from the well. Jugs are capped securely and carried to the field vehicle for further processing. For stream-water samples, dedicated, pre-cleaned (with distilled water in the lab), dried, 2-liter LDPE sampling jugs are rinsed with stream water and the rinse water discarded downstream or on the stream bank. Samples are collected by orienting the jug mouth upstream and submerging it until the bottle fills. Sample is intended to collect moving water whenever possible, so that precise sampling location may vary by ±1 m up and down the stream. After they are filled, jugs are capped securely and carried to the field vehicle for further processing.

After each well sampling, the bailer and suspension wire are rinsed with distilled water. Between each sampling date, the bailer is disassembled and acid washed in the laboratory. Dedicated sampling jugs are rinsed with distilled water between sampling events, and periodically acid washed using 5% HCl.

At the field vehicle, samples are filtered through 0.45 micron filters using a peristaltic pump. For low suspended-solids samples, disposable filters (e.g., Millipore HAWP 0.45-micron filter disks) installed in Teflon housings or disposable low-capacity 0.45µ cartridge filters, or small-capacity 0.45µ disposable cartridges are used. For high suspended-solids samples, disposable high-capacity 0.45µ cartridge filters are used (e.g., Gelman Groundwater Sampling Capsule). Filtered samples are collected in acid-washed 250 milliliter (mL) low-density polyethylene (LDPE) bottles: one bottle is filled to capacity and a second bottle, pre-weighed, is filled with approximately 250 mL of sample. Bottle lids are sealed with Parafilm, placed in pre-chilled reflective cold-storage bags and the bags are stored in a chilled ice chest for transport to the laboratory.

Field pH is measured on the first aliquot collected from the bailer through the bottom- emptying device into pre-cleaned, dedicated, 60-mL narrow-mouth LDPE bottles. Bottles are filled from the bottom to overflowing; a rinsed (distilled-deionized water) pH electrode (whose calibration against buffer solutions with nominal pH of ~4 and ~7 is verified just before and just after sample is collected) is inserted into the bottle, displacing water to waste. The cap mounted on the pH electrode is screwed securely onto the 60-mL bottle, rendering it airtight. The bottle with electrode is placed in insulating material to minimize temperature change. pH and water temperature are recorded after the pH and mV reading has not changed for at least 10 seconds.

2020 to present: Water samples are collected approximately every 4 to 6 weeks or as weather permits from up to 11 wells and 5 surface water sites using methods that are largely consistent with those used in prior years. All wells are sealed with a PVC cap aside from when they are being sampled. To collect samples from wells, we remove the cap and measure the depth-to-water and total depth of each well using a Solinst water level meter. We then bail each well until the saturated volume of water had been removed at least two times. Once the wells are purged, we collect additional bails of water and discharged them into a beaker and a 1 L graduated cylinder for compositional analysis. Prior to discharging the water, we added a bottom emptying device to the bailer to minimize gas exchanges while discharging the sample. We measure field parameters (pH, temperature, conductivity, and dissolved oxygen (DO) concentration) from water in the graduated cylinder while it is overflowing with fresh sample. We draw samples from the beaker for laboratory analysis of major anions and cations and water stable isotopes. Those samples are filtered with 0.45 µm syringe filters and discharged directly into storage bottles. We stored samples in 30 or 60 HDPE bottles for alkalinity and major cation and anion analyses, clear 2 mL glass vials for water isotope samples, and 60 mL amber glass bottles for non-purgeable organic carbon (NPOC) and total dissolved nitrogen (TDN). Bottles for NPOC and TDN are cleaned prior to use by soaking in a 2% hydrochloric acid solution, rinsing with ultrapure water (18 MΩ), and then baking at 250°C overnight. To collect stream samples, we followed identical procedures except no bailing is needed and water samples are drawn directly from the stream channel and probes for field parameters are placed in the stream water column. We measure field parameters for both groundwater and surface water using an Oakton PC 450 pH and conductivity probes and a Professional Series YSI Pro 2030 DO probe. We calibrate each probe before use and rinse them with deionized water between sampling sites.

Laboratory Methods
1990 through 2019: Bottles are transferred to a ~7 degree refrigerator at the end of the field day. The pre-weighed bottles filled with ~250 mL of water are weighed. Concentrated nitric acid is added in the proportion of 1 mL nitric acid for every 50 mL of sample.The bottle is weighed after acid addition and lids are re-sealed with Parafilm for storage until analysis.This is the acidified sample.
One 50-mL aliquot from each unacidified, filtered, full bottle (unacidified sample) is removed for the alkalinity titration. Alkalinity is titrated in the laboratory using 0.02 N H2SO4; the end point is determined by the slope method and checked against the Gran titration method. The initial pH of this sample (before addition of any titrant) is recorded as laboratory pH. Beginning in 2015, alkalinty has also been determined using a 855 Robotic Titrosampler autotitrator.
A ~5-mL aliquot of each unacidified sample is used for anion determination by ion chromatography. A ~25 mL aliquot of each acidified sample is used for determination of cations by ICP-OES. The analytical techniques are summarized briefly below.

1991 - 2016: F, Cl, NO3-N, PO4-P, and SO4 are determined by ion chromatography with a Dionex 4000i ion chromatograph (EPA Method 300.0). Analysis is accomplished by suppressed conductivity detection using IONPAC AS4A-SC separator column, IONPAC AG4A-SC guard column, and an anion self-regenerating suppressor. Eluent is 1.8 mM Na2CO3 and 1.7 mM NaHCO3 pumped at a rate of 2 mL/min. The suppressor is continuously regenerated with distilled-deionzed water. The sample loop size is 25 microliters; 5 mL of sample is spiked with 50 microliters of 100X eluent to minimize the water dip interference with F and Cl determination. Samples are analyzed twice and the average of the two analyses reported as long as the difference between the two is less than 3% of the lower value. Quality control samples from various sources are used to check accuracy of the determinations.

2016 - present: F, Cl, NO3-N, PO4-P, and SO4 are determined by ion chromatography with a BRAND MODEL (EPA Method 300.0). Analysis is accomplished by suppressed conductivity detection using IONPAC AS23 separator column, IONPAC AG23 guard column, and a AERS-500 anion self-regenerating suppressor. Eluent made from Dionex AS 23 Concentrate and after 10x dilution is 4.5 mM Na2CO3 and 0.8 mM NaHCO3; eluent is pumped at a rate of 1 mL/min. The suppressor is continuously regenerated with distilled-deionzed water. The sample loop size is 25 microliters; 5 mL of sample is used for the analysis. Samples are analyzed twice and the average of the two analyses reported as long as the difference between the two is less than 3% of the lower value. Quality control samples from various sources are used to check accuracy of the determinations.
ICP-OES (Instruments SA, Inc., JY-138Ultrace) is used to determine dissolved concentrations of Na, K, Li, Ca, Mg, Sr, Ba, Si, and B using a ~25 mL aliquot from the acidified sample. All determinations are made in duplicate and checked against quality control samples from various sources and/or against Standard Reference Materials water samples from the National Institute of Standards and Technology (NIST).

May 2018 - Dec 2018: For AGW011 data: From May through December 2018 water chemistry samples, there was a long holding time for anion determination (SO4, Cl, F, NO3-N) due to equipment failure, making the reported data potentially unreliable.

2020 to present: We analyze alkalinity and major ion concentrations for all samples within a month of collection in the Geology Department and Division of Biology at Kansas State University. Water samples are stored in a laboratory refrigerator until they are analyzed. Cation samples are preserved by adding trace-metal grade nitric acid to a final volume of 0.2%. Similarly, NPOC and TDN samples are preserved by adding concentrated hydrochloric acid to a final volume of 0.2%. For alkalinity measurements, we have used two techniques. During samples collected from Oct. 31, 2021 to Feb. 27, 2022, we used Gran alkalinity titrations with a glass burette and 0.02 N sulfuric acid titrant. Then, for the remainder of the samples, we used end-point (pH 4.5) titrations using a ThermoScientific OrionStarT910 pH Titrator again with 0.02 N sulfuric acid titrant. These methods gave consistent results when performed on replicate samples. We measure anion (F-, Cl-, NO3-, SO42-) and cation concentrations (Na+, K+, Mg2+, Ca2+, Sr2+) using Thermo Scientific ICS-1100 Ion Chromatographs. We measured NPOC and TDN using a Shimadzu TOC-L. For IC and TOC-L analyses, quality control procedures include requiring a correlation coefficient of at least 0.995 for successful calibration and periodic analysis of calibration verification samples during each run. We measured the ẟ18O and ẟD of water isotope samples at 0.1 and 0.5‰ precision, respectively, using a Picarro L-i2130 water analyzer. Results are expressed in delta notation relative to Vienna Standard Mean Ocean Water (VSMOW).

Form of Data Output
All data are recorded onto a computer spreadsheet (Microsoft Excel). Several derivative properties are calculated by the spreadsheet. These include total dissolved solids, hardness, milliequivalents of cations and anions, and charge balance.

Summary of All Changes: From 1991 through 1993, all wells that contained water were sampled and water chemistry determined. From 1993- only a subset of wells have been sampled for water chemistry.
Jan 1991 - April 1994: Cations measured were Na, K, Ca, Mg by AAS
April 1994 - May 1997:Cations measured wee Na, K, NH4-H, Ca, Mg by IC
April 1998- April 2000: Si also determined by the molydosilicate method (Standard Methods #426B) using a Spectonix 2000 UV-VIS spectrometer
2020 to present – the monitoring effort started by Dr. Gwen Macpherson was picked up by Dr. Matthew Kirk.

For additional metadata information see: http://lter.konza.ksu.edu/sites/default/files/DC.pdf

For additional methods information see: http://lter.konza.ksu.edu/sites/default/files/MM.pdf