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Two water‐quality studies were done on the outskirts of the Detroit metropolitan area to determine how recent residential development has affected ground‐water quality. Pairs of monitor and domestic wells were sampled in areas where residential land use overlies glacial outwash deposits. Young, shallow waters had significantly higher median concentrations of nitrate, chloride, and dissolved solids than older, deeper waters. Analysis of chloride/bromide ratios indicates that elevated salinities are due to human activities rather than natural factors, such as upward migration of brine. Trace concentrations of volatile organic compounds were detected in samples from 97 percent of the monitor wells. Pesticides were detected infrequently even though they are routinely applied to lawns and roadways in the study area. The greatest influence on ground‐water quality appears to be from septic‐system effluent (domestic sewage, household solvents, water‐softener backwash) and infiltration of storm‐water runoff from paved surfaces (road salt, fuel residue). No health‐related drinking‐water standards were exceeded in samples from domestic wells. However, the effects of human activities are apparent in 76 percent of young waters, and at depths far below 25 feet, which is the current minimum well‐depth requirement.

Ground water systems dominated by iron- or sulfate-reducing conditions may be distinguished by observing concentrations of dissolved iron (Fe(2+)) and sulfide (sum of H(2)S, HS(-), and S(=) species and denoted here as "H(2)S"). This approach is based on the observation that concentrations of Fe(2+) and H(2)S in ground water systems tend to be inversely related according to a hyperbolic function. That is, when Fe(2+) concentrations are high, H(2)S concentrations tend to be low and vice versa. This relation partly reflects the rapid reaction kinetics of Fe(2+) with H(2)S to produce relatively insoluble ferrous sulfides (FeS). This relation also reflects competition for organic substrates between the iron- and the sulfate-reducing microorganisms that catalyze the production of Fe(2+) and H(2)S. These solubility and microbial constraints operate in tandem, resulting in the observed hyperbolic relation between Fe(2+) and H(2)S concentrations. Concentrations of redox indicators, including dissolved hydrogen (H(2)) measured in a shallow aquifer in Hanahan, South Carolina, suggest that if the Fe(2+)/H(2)S mass ratio (units of mg/L) exceeded 10, the screened interval being tapped was consistently iron reducing (H(2) approximately 0.2 to 0.8 nM). Conversely, if the Fe(2+)/H(2)S ratio was less than 0.30, consistent sulfate-reducing (H(2) approximately 1 to 5 nM) conditions were observed over time. Concomitantly high Fe(2+) and H(2)S concentrations were associated with H(2) concentrations that varied between 0.2 and 5.0 nM over time, suggesting mixing of water from adjacent iron- and sulfate-reducing zones or concomitant iron and sulfate reduction under nonelectron donor-limited conditions. These observations suggest that Fe(2+)/H(2)S mass ratios may provide useful information concerning the occurrence and distribution of iron and sulfate reduction in ground water systems.

A better understanding of public-supply-well vulnerability to contamination can lead to a reduced need for treatment of drinking water. Well screen Well casing Public-supply well Recharge Monitoring wells Cone of depression Domestic well Water table Groundwater flow pathways Chapter 1-Major Findings and Implications 3 dispersion of contaminants as they move through an aquifer. Third is the ease with which water and contaminants can travel to and through an aquifer, also referred to as intrinsic susceptibility; for example, a thick layer of dense clay can reduce groundwater vulnerability by acting as a barrier to the movement of water and contaminants. The vulnerability of a public-supply well depends on all of the above factors (contaminant input, contaminant mobility and persistence, and intrinsic susceptibility) but is further affected by the location, design, construction, operation, and maintenance of the well. For example, the location of a well determines whether a particular contaminant source is in the area that contributes water to the well. The placement of the well screen determines which chemical and physical processes in the aquifer have influenced the water before it is pumped from the well and, therefore, which contaminants might be present in the water as it enters the well, and at what concentrations. The depth of the well screen and the pumping rate of the well determine how quickly water and contaminants can travel from the water table to that particular well, and from what distance. Because well design, construction, and operation directly influence water quality, water produced by different types of wells (public-supply, domestic, and monitoring wells) might not contain the same concentrations of contaminants, even if the wells are completed at similar depths within the same aquifer. What Measures of Vulnerability are Useful for Individual Public-Supply Wells? Study findings indicate that information on contaminant input, contaminant mobility and persistence, and intrinsic susceptibility within the area that con tributes water to a well can help answer the question, "Which contaminants in an aquifer might reach the well, and when, how, and at what concentration might they arrive?" Study-team scientists found that the following measures-each related to a different aspect of public-supply-well vulnerability-are particularly useful for understanding the quality of water pumped from individual publicsupply wells: (1) the sources of recharge that contribute water to a well, and the contaminants associated with the recharge-a measure of contaminant input; (2) the geochemical conditions encountered by water drawn into a well-a measure of contaminant mobility and persistence; and (3) the groundwater-age mixture of different waters that blend (or mix) in a well-a measure of intrinsic susceptibility. These measures of public-supply-well vulnerability and examples illustrating their utility for decisionmaking are discussed herein. Related implications for public-supply-well water quality are noted. Wate...

These professionals contributed to (1) planning and oversight of the project, and (2) compilation and review of multiagency arsenic-related data for a Web site (http://ArsenicInOhioGroundwater.info). The author would also like to thank Chad Brown and his staff at the Licking County Health Department for: (1) collection of water samples for analysis of arsenic during routine well inspection activities, and (2) efforts related to planning and implementation of the Test Your Well workshops. The author also appreciates assistance with Test Your Well workshops from: members of the advisory committee (listed above); Curtis Coe