Ground-Water Chemistry (AGW01)

Purpose: To study temporal variations in shallow ground-water chemistry in a pristine 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 the nitrogen cycle below ground at that watershed.

Location of Sampling Station: Thirty-nine wells are located in the lower third of the N04d watershed (see Fig. 1). Water levels are measured on all wells. Inorganic chemistry of ground water from up to seven wells and two stream sampling sites is determined.

Frequency of Sampling: Samples are collected every 4-6 weeks, as weather permits.

Variables Measured: 1) Depth to water (water level), all wells. 2) Concentrations of Ca, Mg, Na, K, Si, Al, B, Sr, Ba, Fe, Mn, Zn, Li, NO3-N, HPO4-P, SO4, Cl, alkalinity in water samples.

Field Methods: Water level measurement Depths to water and to the bottom of the wells are measured, after removing the PVC well cap and allowing the well to "breathe" for several minutes, using a water level meter (e.g., Solinist 101, with flat polyethylene cable and stainless steel probe). Reference points are marked on the well casings. Reproducibility of measurements is on the order of 6 mm (0.02 feet).

Water chemistry 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 TeflonŽ bailer suspended on TeflonŽ-coated steel wire. Samples are emptied into dedicated, rinsed (with ground water), 2-liter, low-density polyethylene (LDPE) jugs using a bottom-emptying device inserted into the bailer after it is removed from the well. Jugs are capped securely and immediately carried to the field vehicle (maximum transport time is about 10 minutes) for further processing. The bailer is not touched by human skin during the sampling process.

Stream-water samples are collected by first rinsing dedicated, 2-liter, LDPE sampling jugs with stream water and dumping the rinse water downstream. Samples are collected by orienting the jug mouth upstream and submerging it until the bottle fills. Sample sites are selected to collect moving water whenever possible. Jugs are capped securely and immediately carried to the field vehicle (maximum transport time is about 10 minutes) for further processing.

Between sampling events the bailer and suspension wire is rinsed thoroughly with distilled water. The bailer is periodically disassembled and acid washed. 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 filter disks (e.g., MilliporeŽ HAWP 0.45 micron filter disks) installed in TeflonŽ housings are used. For high suspended-solids samples, disposable high capacity cartridge filters are used (e.g., Gelman Groundwater Sampling Capsule or equivalent). Filtered samples are collected in acid-washed 250 milliliter (mL) low-density polyethylene (LDPE). One LDPE bottle is filled to capacity. A second LDPE bottle, preweighed, is filled with approximately 250 mL of sample. Bottles are stored in an ice chest for transport to the laboratory and then stored in a refrigerator.

Laboratory Methods: At the laboratory, the 250-mL bottles are weighed and then 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.

One 50-mL aliquot from each unacidified sample is used for the alkalinity titration. Alkalinity is titrated in the laboratory using 0.02 N H2SO4; the end point is determined by the slope method. The pH of this sample is recorded as laboratory pH.

One 5-mL portion of the unacidified sample is used for anion determination by ion chromatography. The acidified aliquot of each sample is used for determination of cations by ICP-AES. The analytical techniques are summarized briefly below.

F, Cl, NO2-N, NO3-N, 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 supressor. Eluent is 1.8 mM Na2CO3 and 1.7 mM NaHCO3 at 2 mL/min. The suppressor is continuously regenerated with distilled-deionzed water. The sample loop is 25 microliters; 5 mL of sample is spiked with 50 microliters of 100 times concentration 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.

ICP-AES (using an Instruments SA, Inc. JY-138Ultrace) is used to determine concentrations of Na, K, Ca, Mg, Li, Si, B, Sr, Ba, Fe, Al, Zn and Mn using 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 up to 2002:  From 1991 through 1993, all wells that contained water were sampled and water chemistry determined. Jan 1991 - April 1994: Na, K, Ca, Mg by AAS April 1994 - May 1997: Na, K, NH4-H, Ca, Mg by IC May 1997 - present: Na, K, Li, Ca, Mg, Sr, Ba, Zn, Fe, Mn, Al, B, Si by ICP-AES April 1998- April 2000: Si also by the molydosilicate method (Standard Methods #426B) using a Spectonix 2000 UV-VIS spectrometer