Denitrification and dilution along fracture flowpaths influence the recovery of a bedrock aquifer from nitrate contamination

•Bedrock wells contaminated with nitrates at a dairy farm in Vermont, U.S.A.•Nitrate concentration vs. time patterns for wells were spatially separable.•Multidisciplinary aquifer characterization used physical and chemical methods.•Denitrification dominant over dilution along fracture flowpaths•Conceptual model shows exhaustion of a nitrate point-source over 12 years.

In 2000, elevated nitrate concentrations ranging from 12 to 34 mg/L NO3N were discovered in groundwater from numerous domestic bedrock wells adjacent to a large dairy farm in central Vermont. Long-term plots and contours of nitrate vs. time for bedrock wells showed “little/no”, “moderate”, and “large” change patterns that were spatially separable. The metasedimentary bedrock aquifer is strongly anisotropic and groundwater flow is controlled by fractures, bedding/foliation, and basins and ridges in the bedrock surface. Integration of the nitrate concentration vs. time data and the physical and chemical aquifer characterization suggest two nitrate sources: a point source emanating from a waste ravine and a non-point source that encompasses the surrounding fields. Once removed, the point source of NO3 (manure deposited in a ravine) was exhausted and NO3 dropped from 34 mg/L to < 10 mg/L after ~ 10 years; however, persistence of NO3 in the 3 to 8 mg/L range (background) reflects the long term flux of nitrates from nutrients applied to the farm fields surrounding the ravine over the years predating and including this study. Inferred groundwater flow rates from the waste ravine to either moderate change wells in basin 2 or to the shallow bedrock zone beneath the large change wells are 0.05 m/day, well within published bedrock aquifer flow rates. Enrichment of 15N and 18O in nitrate is consistent with lithotrophic denitrification of NO3 in the presence of dissolved Mn and Fe. Once the ravine point-source was removed, denitrification and dilution collectively were responsible for the down-gradient decrease of nitrate in this bedrock aquifer. Denitrification was most influential when NO3N was > 10 mg/L. Our multidisciplinary methods of aquifer characterization are applicable to groundwater contamination in any complexly-deformed and metamorphosed bedrock aquifer.

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