Methods Used to Assess the Susceptibility to Contamination of Transient, Non-Community Public Ground-Water Supplies in Indiana

By Leslie D. Arihood and David A. Cohen

Prepared in cooperation with the Indiana Department of Environmental Management

ONLINE ONLY

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Abstract

The Safe Water Drinking Act of 1974 as amended in 1996 gave each State the responsibility of developing a Source-Water Assessment Plan (SWAP) that is designed to protect public-water supplies from contamination. Each SWAP must include three elements: (1) a delineation of the source-water protection area, (2) an inventory of potential sources of contaminants within the area, and (3) a determination of the susceptibility of the public-water supply to contamination from the inventoried sources. The Indiana Department of Environmental Management (IDEM) was responsible for preparing a SWAP for all public-water supplies in Indiana, including about 2,400 small public ground-water supplies that are designated transient, non-community (TNC) supplies. In cooperation with IDEM, the U.S. Geological Survey compiled information on conditions near the TNC supplies and helped IDEM complete source-water assessments for each TNC supply.

The delineation of a source-water protection area (called the assessment area) for each TNC ground-water supply was defined by IDEM as a circular area enclosed by a 300-foot radius centered at the TNC supply well. Contaminants of concern (COCs) were defined by IDEM as any of the 90 contaminants for which the U.S. Environmental Protection Agency has established primary drinking-water standards. Two of these, nitrate as nitrogen and total coliform bacteria, are Indiana State-regulated contaminants for TNC water supplies. IDEM representatives identified potential point and nonpoint sources of COCs within the assessment area, and computer database retrievals were used to identify potential point sources of COCs in the area outside the assessment area.

Two types of methods—subjective and subjective hybrid—were used in the SWAP to determine susceptibility to contamination. Subjective methods involve decisions based upon professional judgment, prior experience, and (or) the application of a fundamental understanding of processes without the collection and analysis of data for a specific condition. Subjective hybrid methods combine subjective methods with quantitative hydrologic analyses.

The subjective methods included an inventory of potential sources and associated contaminants, and a qualitative description of the inherent susceptibility of the area around the TNC supply. The description relies on a classification of the hydrogeologic and geomorphic characteristics of the general area around the TNC supply in terms of its surficial geology, regional aquifer system, the occurrence of fine- and coarse-grained geologic materials above the screen of the TNC well, and the potential for infiltration of contaminants. The subjective hybrid method combined the results of a logistic regression analysis with a subjective analysis of susceptibility and a subjective set of definitions that classify the thickness of fine-grained geologic materials above the screen of a TNC well in terms of impedance to vertical flow. The logistic regression determined the probability of elevated concentrations of nitrate as nitrogen (greater than or equal to 3 milligrams per liter) in ground water associated with specific thicknesses of fine-grained geologic materials above the screen of a TNC well. In this report, fine-grained geologic materials are referred to as a geologic barrier that generally impedes vertical flow through an aquifer. A geologic barrier was defined to be thin for fine-grained materials between 0 and 45 feet thick, moderate for materials between 45 and 75 feet thick, and thick if the fine-grained materials were greater than 75 feet thick.

A flow chart was used to determine the susceptibility rating for each TNC supply. The flow chart indicated a susceptibility rating using (1) concentrations of nitrate as nitrogen and total coliform bacteria reported from routine compliance monitoring of the TNC supply, (2) the presence or absence of potential sources of regulated contaminants (nitrate as nitrogen and coliform bacteria) within the assessment area, and (3) the thickness of the geologic barrier above the TNC well screen. The possible susceptibility ratings were: "currently not susceptible," "moderately susceptible," or "susceptible," A rating of susceptible was automatically given to a TNC supply if there was a detection of coliform bacteria or a concentration of nitrate as nitrogen greater than 3 mg/L in any compliance-monitoring sample. Less than 2 percent (43) of the TNC supplies could not be rated because they were new and no compliance-monitoring data were available. Only one of the TNC supplies was rated not currently susceptible, approximately 7 percent (164) were rated moderately susceptible, and approximately 91 percent (2,144) were rated susceptible. Of the 2,144 TNC supplies rated susceptible, approximately 79 percent (1,694) had a detection of coliform bacteria or a concentration of nitrate as nitrogen greater than 3 mg/L in at least one compliance-monitoring sample.

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Contents

Abstract

Introduction

Indiana’s Source-Water Assessment Plan

Purpose and Scope

Methods Used to Assess the Susceptibility to Contamination

Methods Used to Qualitatively Describe Susceptibility

Presence of Potential Sources and Associated Contaminants of Concern

Hydrogeologic and Geomorphic Descriptions

Methods Used to Determine a Susceptibility Rating

Summary

References Cited

Appendixes 1–3

1. Example Transient, Non-Community Source-Water Assessment Report


2. Potential Sources of Drinking-Water Contamination Index


3. Concentrations of Nitrate and Thickness of Fine-Grained Geologic Materials Above the Well Screen in Indiana

1. Transient, non-community public-water supplies in Indiana

2. Location and extent of unconsolidated regional aquifers in Indiana

3. Location and extent of consolidated regional aquifers in Indiana

4. Relation between concentrations of nitrate as nitrogen in ground water and the thickness of fine-grained geologic materials above the well screen in Indiana

5. Probability of nitrate as nitrogen in ground water being greater than or equal to 3 milligrams per liter for various thicknesses of fine-grained geologic materials above the well screen in Indiana