National Water-Quality Assessment (NAWQA) Program

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Water Quality in Carbonate Aquifers in the United States

The Report Summary of Findings Study Design

Summary of Findings

Carbonate aquifers may have karst features such as sinkholes and caves, which can make these systems vulnerable to contamination. Several factors, however, can limit contamination. Examples of these factors include undeveloped land overlying the aquifers, and characteristics of the aquifer such as confinement and geochemical conditions limiting the mobility and transport of contaminants. However, in aquifers lacking protective features and having sources of contaminants at the land surface, elevated concentrations of contaminants are likely. In carbonate aquifers without these protective features and with sources of contaminants at the surface, concentrations of contaminants are among the highest in the nation.

Human Health

NITRATE: Samples from 54 of 1,037 sites (5 percent) exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) of 10 mg/L as nitrogen (N) for nitrate in drinking water

PESTICIDES: Of 1,027 sites sampled for 47 pesticides (45 with human health benchmarks), samples from 21 sites exceeded human-health benchmarks.

VOLATILE ORGANIC COMPOUNDS: Of 793 sites sampled for at least 54 volatile organic compounds (38 with human health benchmarks), samples from 5 sites exceeded human-health benchmarks.

Findings on factors affecting concentrations or detections of contaminants:

Human activities at the land surface such as agricultural or urban land use, affect the concentrations and detection frequency of human made contaminants.

Natural physical and chemical characteristics of the aquifers

The natural physical and chemical characteristics of the aquifers affect the concentrations and detection frequency of contaminants. Features include climate, aquifer depth, presence of confining layers, and the development of karst features. The extent to which these features impede or increase the speed water travels through the aquifer, and enhance or impede attenuation of contaminants, affects the concentrations and frequency of detection at the sampling point.

Combination of the land surface activities and the physical and chemical properties

The combination of the land surface activities and the physical and chemical properties of the aquifer provide the best explanation of the results of the study.

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