|Scientific Investigations Report 2006-5281|
By Paul M. Buszka, Lee R. Watson, and Theodore K. Greeman
Prepared in cooperation with the City of Richmond, Indiana
Assessments of the vulnerability to contamination of ground-water sources used by public-water systems, as mandated by the Federal Safe Drinking Water Act Amendments of 1996, commonly have involved qualitative evaluations based on existing information on the geologic and hydrologic setting. The U.S. Geological Survey National Water-Quality Assessment Program has identified ground-water-age dating; detailed water-quality analyses of nitrate, pesticides, trace elements, and wastewater-related organic compounds; and assessed natural processes that affect those constituents as potential, unique improvements to existing methods of qualitative vulnerability assessment. To evaluate the improvement from use of these methods, in 2002 and 2003, the U.S. Geological Survey, in cooperation with the City of Richmond, Indiana, compiled and interpreted hydrogeologic data and chemical analyses of water samples from seven wells in a part of the Whitewater Valley aquifer system in a former glacial valley near Richmond. This study investigated the application of ground-water-age dating, dissolved-gas analyses, and detailed water-quality analyses to quantitatively evaluate the vulnerability of ground water to contamination and to identify processes that affect the vulnerability to specific contaminants in an area of post-1972 greenfield development.
The aquifer system in the study area includes an unconfined sand and gravel aquifer used for public-water supply (upper aquifer) and a confined sand and gravel aquifer (lower aquifer) separated by a till confining unit. Several hydrogeologic and cultural measures indicate that the upper aquifer is qualitatively vulnerable to contamination: the upper aquifer is unconfined and has a shallow depth to the water table (from about 4.75 to 14 feet below land surface), low-permeability sediments in the unsaturated zone are thin (less than 10 feet thick), estimated ground-water-flow rates through the upper aquifer are relatively rapid (the highest estimated rates ranged from 0.44 to about 5.0 feet per day), and potential contaminant sources were present.
Ground-water-age dates indicate that ground-water samples represented recharge from about the time greenfield development began south of the ground-water-flow divide and that changes in water quality would lag changes in contaminant inputs. Estimates of ground-water age, computed with dichlorodifluoromethane (CFC-12) and trichlorotrifluoroethane (CFC-113) concentrations in water samples collected from seven observation wells in February and March 2003, indicated that water in the upper aquifer had recharged within about 13 to 30 years before sampling. Ground-water ages were youngest (from about 13 to 15 years since recharge) in water from the shallow wells along the glacial-valley margin and oldest (30 years) in water from a well at the base of the aquifer in the valley center. Ground-water ages determined for the shallow wells may be affected by mixing of recent recharge with older ground water from deeper in the aquifer, as indicated by upward hydraulic gradients between paired shallow and deep wells in the upper aquifer. Other parts of the Whitewater Valley aquifer system with similar hydrogeologic characteristics could be expected to have similarly young ground-water ages and residence times.
Analyses of water samples collected from the seven observation wells in August and September 2002 indicated that concentrations of chloride, sodium, and nitrate generally were larger in ground water from the upper aquifer than in other parts of the Whitewater Valley aquifer system. Drinking-water-quality standards for Indiana were exceeded in water samples from one well for chloride concentrations, from four wells for dissolved-solids concentrations, and from one well for nitrate concentrations. Application of low-level methods for trace-element analyses determined that concentrations of aluminum, cobalt, iron, lithium, molybdenum, nickel, selenium, uranium, vanadium, and zinc were less than or equal to 8 micrograms per liter; concentrations of arsenic, cadmium, chromium, and copper were less than or equal to 1 microgram per liter. Application of low-level analytical methods to water samples enabled the detection of several pesticides and volatile, semivolatile, and wastewater-related organic compounds; concentrations of individual pesticides and volatile organic compounds were less than 0.1 microgram per liter and concentrations of individual wastewater organic compounds were less than 0.5 microgram per liter. The low-level analytical methods will provide useful data with which to compare future changes in water quality.
Results of detailed water-quality analyses, ground-waterage dating, and dissolved-gas analyses indicated the vulnerability of ground water to specific types of contamination, the sequence of contaminant introduction to the aquifer relative to greenfield development, and processes that may mitigate the contamination. Concentrations of chloride and sodium and chloride/bromide weight ratios in sampled water from five wells indicated the vulnerability of the upper aquifer to roaddeicer contamination. Ground-water-age estimates from these wells indicated the onset of upgradient road-deicer use within the previous 25 years. Nitrate in the upper aquifer predates the post-1972 development, based on a ground-water-age date (30 years) and the nitrate concentration (5.12 milligrams per liter as nitrogen) in water from a deep well. Vulnerability of the aquifer to nitrate contamination is limited partially by denitrification. Detection of one to four atrazine transformation products in water samples from the upper aquifer indicated biological and hydrochemical processes that may limit the vulnerability of the ground water to atrazine contamination. Microbial processes also may limit the aquifer vulnerability to small inputs of halogenated aliphatic compounds, as indicated by microbial transformations of trichlorofluoromethane and trichlorotrifluoroethane relative to dichlorodifluoromethane. The vulnerability of ground water to contamination in other parts of the aquifer system also may be mitigated by hydrodynamic dispersion and biologically mediated transformations of nitrate, pesticides, and some organic compounds. Identification of the sequence of contamination and processes affecting the vulnerability of ground water to contamination would have been unlikely with conventional assessment methods.
Purpose and Scope
Description of the Study Area
Methods of Data Collection and Analysis
Well-Site Selection and Installation
Ground-Water-Quality Sampling and Analyses
Quality-Assurance Sampling and Analyses
Estimation of Ground-Water Age
Hydrogeology of a Part of the Whitewater Valley Aquifer System
Dissolved-Gas Concentrations and Estimated Average Recharge Temperatures
Chlorofluorocarbon-Based Ground-Water Ages
Sulfur Hexafluoride Data
Evaluation of Quality-Assurance Data
General Ground-Water Chemistry, Major Cations and Anions, and Alkalinity
Pesticides and Organic Compounds
Vulnerability of Ground Water to Contamination
Summary and Conclusions
1.–6. Maps showing—
|1.||Study area near Richmond, Indiana, relative to the Whitewater Valley aquifer
system and major streams, Indiana and Ohio.
|2.||Location of the study area, the approximate extent of coarse-grained sediments
of the Whitewater Valley aquifer system, the former glacial valley and upper
aquifer, and the Aquifer Protection District near Richmond, Indiana.
|3.||Location of the study area relative to selected surface watersheds, the
upper aquifer, and the area of detailed study.
|4.||Developed areas, structures, and land uses in the study area relative to the
upper aquifer and observation wells installed for this study, 2003.
|5.||Selected wells used to interpret extent and thickness of the upper aquifer
and wells drilled into the lower aquifer or the Fayette-Union aquifer system.
|6.||Area of detailed study with observation wells installed for this study, selected
other wells, and lines of hydrogeologic sections A-A’ and B-B’ through the
|7.||Graph showing atmospheric concentrations of dichlorodifluoromethane,
trichlorofluoromethane, trichlorotrifluoroethane, and sulfur hexafluoride for air in
|8.||Map showing extent and thickness of the upper aquifer and selected wells that
penetrate the upper aquifer.
|9.–10.||Hydrogeologic sections showing the upper and lower aquifers, types of sediment
and bedrock encountered, and approximate water-table and potentiometric-
|9.||Section A-A', oriented south to north across the study area.|
|10.||Section B-B’, oriented west to east across the study area.|
|11.||Borehole geophysical logs of natural gamma radiation from observation wells 1-23,
2-26, and 3-38; the distribution of fine- and coarse-grained sediments; and the
altitude of well-screen intervals of these and selected nearby observation wells in
the upper aquifer.
|12.–13.||Maps showing the altitude of the water table in the upper aquifer, relative to the approximate area of a ground-water-flow divide-|
|12.||December 24, 2002.|
|13.||April 10, 2003.|
|14.||Graph showing concentrations of dissolved nitrogen and dissolved argon in water
samples and sequential duplicates from the upper aquifer, 2002 data, plotted on a
grid with hypothetical concentrations of dissolved nitrogen and dissolved argon at
various recharge temperatures and concentrations of excess air in water samples.
|15.||Stiff diagrams showing the relation of concentrations of major cations and anions
in water-quality samples from the upper aquifer and in a sequential duplicate,
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