Inorganic Nutrients

AGW01 Long-term measurement of groundwater physical and chemical properties from wells on watershed N04D at Konza Prairie

Abstract: 

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. The wells range in depth from about 2 to 13 meters, and are nested to include wells completed in alluvium/colluvium near the N04D drainage and in two Permian-aged limestone’s. In 1997, Konza LTER drilled an additional 4 wells at 3 sites, that range in depth from about 12 m to 37 m. The sites comprise four transects running approximately east-west across the drainage, and occupy the lower one-quarter of the surface area of the watershed. The geology of the area is characterized by patchy, near-stream alluvium/colluvium which overlies bedrock that is composed of thin (1-2 meter) limestones alternating with thicker (2-4 meter) shales. Beginning in 1990, water levels were measured and water samples collected quarterly. Since 1991, water levels have been measured and water samples for inorganic chemical analysis have been collected every four to six weeks; sample collection is by bailing wells after removing 1-2 well volumes of groundwater. The groundwater is then filtered through 0.45 micro; membrane filters by gravity feed or using a peristaltic pump. One 250-mL LDPE bottle is filled with 250 mL of sample and 5 mL of concentrated HNO3 for determination of major and minor cation concentrations. One 250-mL LDPE bottle is filled to the brim for anion determination by ion chromatography and alkalinity determination by titration with 0.02 N H2SO4. The bottles are stored on ice and later in a refrigerator.

Core Areas: 

Data set ID: 

1

Short name: 

AGW01

Purpose: 

To study temporal variations in shallow groundwater chemistry in an unplowed tallgrass prairie watershed; to examine stream-aquifer interactions in that watershed; to calculate chemical denudation rates of a prairie setting underlain by limestone and shale; to examine sources and sinks of dissolved species in groundwater at that watershed.

Methods: 

Location of Sampling Stations: Thirty-five wells are located in the lower third of the N04d watershed, approximately along four linear transects perpendicular to the South Fork of Kings Creek.

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 drainage, and occupy the lower one-quarter of the surface area of the watershed. Also sampled are 2 to 4 of the locations where the transects cross the stream.

Frequency of Sampling: 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 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. Depths 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.

Laboratory Methods: 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).

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

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

Data sources: 

Maintenance: 

ongoing

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