Page Banner

United States Department of Agriculture

Agricultural Research Service

Title: Spatial Variability of Chemical Constituents in Ground Water over Short Distances

Author
item Bonta, James

Submitted to: International Conference on Water and Irrigation
Publication Type: Proceedings
Publication Acceptance Date: March 2, 2000
Publication Date: N/A

Interpretive Summary: Ground-water monitoring is expensive and difficult to do correctly. Typically, only a few ground-water monitoring wells are used and the assumption is made that these few data are representative of the water quality in the aquifer. However, the assumption is seldom tested because of the expense involved. An isolated hilltop was instrumented with a dense network of ground-water monitoring sites and samples collected. The study documented the highly variable nature of sulfate and nitrate ion concentrations in ground water over short distances less than 50 m. It was found that stable estimates of average, median and variability of these ions is possible with about 10 wells, with a density of about 3.1 wells/hectare. Little improvement was apparent with more than 10 wells. The results are useful for documenting the variability of measured ground-water data and for guidance on the number of wells needed to represent the quality of water in an aquifer, even over short distances. This is important because ground-water quality model parameters can be distorted when trying to match measured water quality and the high degree of uncertainty in model validation data is documented by this study. Furthermore, the uncertainty in water quality data helps in interpretation and extrapolation of the little data that are often available. The study has economic and public health implications also when interpreting ground-water data. Users of the information include Federal, state and university scientists, consultants and regulatory agencies.

Technical Abstract: The expensive nature of ground-water monitoring precludes intensive monitoring-well installation, sampling, and laboratory analyses in practice. The few ground-water quality measurements often available are assumed to be representative without much thought given to the adequacy of this assumption. A 3.27- ha experimental hilltop in Coshocton, Ohio was instrumented for ground-water monitoring. The spatial variability of sulfate and nitrate concentrations in ground water was investigated. Sulfate concentrations varied up to 27 times over distances shorter than 50 feet, and nitrate varied over 64 times for values greater than zero. Stable estimates of average, median, and coefficient of variation could be obtained if about 10 wells were used, a well density of 3.1 wells/ha. Natural variability of ground water must be taken into account to minimize costs of remediation and to protect ground-water resources.

Last Modified: 10/25/2014
Footer Content Back to Top of Page