GROUNDWATER IDENTITY: ARE WELLS UNIQUE? A CASE STUDY FROM UPSTATE NEW YORK

In advance of high volume hydraulic fracturing (HVHF) activities planned for central New York, we have initiated an inventory of groundwater elemental chemistry in local drinking water wells. We sampled 32 wells and analyzed for anions and 66 metal ions. 16 of the wells were sampled more than once. Most of the wells penetrate unconsolidated sand and gravel and extend to Devonian age sedimentary strata. We find a significant range in elemental chemistry. We present a statistical summary of elemental concentrations, and we introduce a method for comparing wells against each other. The method is simple. The elemental concentrations in one well are plotted against another well, and a power law is fit to the data. The parameters (that is, the coefficient and the exponent) in the power law, along with a measure of the scatter, provide a powerful tool to characterize similarity and uniqueness. When the power law coefficient, correlation coefficient, and power law exponent approach unity, the samples approach identical concentrations. Similarity implies uniform dilution or concentration for all species being compared. When the exponent is approximately unity, the coefficient indicates which sample is more or less concentrated than the other. When the exponent is greater than unity, major elements are more enriched in one well. The correlation coefficient (R², in this case) measures the scatter around the power law relation. As the correlation coefficient approaches 0, a wide scatter exists, even if the exponent indicates similarity. We applied this method of comparing the elemental concentrations in two samples to each other to discover that individual wells look far more like themselves than any other well. One implication is that groundwater flow paths have characteristic chemical reactions with rocks along their path to the well, and reach a steady state concentration. Temporal variations amount to uniform changes in concentrations across all elements, such as might occur from mixing with very fresh water. Any mixing with non-identical water will yield either more scatter, or values for the power law parameters other than unity. We could detect 30 to 45 elements in most wells. Of the 48 samples we collected, 15 major, trace, and rare earth elements were detected in all wells, and provide a local fingerprint of groundwater.