Land use and geologic controls on the major elemental and isotopic (δ15N and 87Sr/86Sr) geochemistry of the Connecticut River watershed, USA

We studied the relationship between land use, geology and water chemistry in the Connecticut River watershed of northeastern USA by combining Geographic Information Systems (GIS) with nitrogen and strontium isotopic analyses and major cation and anion geochemistry. River water was sampled at 35 sites in the summer and fall of 1998 and the spring of 1999 in an attempt to distinguish spatial and temporal changes in geochemical signatures related to land use and bedrock geology. We coupled 87Sr/86Sr values and major element concentrations from river water to quantify the relationship between bedrock geology and riverine geochemistry. The results in our study show that strontium isotopes of tributary waters reflect their catchment bedrock lithology. Our results also suggest that major cations in the Connecticut River water are dominated by carbonate weathering despite the limited presence of calcareous metasedimentary rocks (∼10% of watershed bedrock). In addition, the relative contribution of carbonate to silicate weathering increases downriver as a result of input from tributaries draining Paleozoic carbonate-bearing rocks. In contrast, nitrate concentrations and nitrogen isotopic ratios of nitrate in water were highly variable. Nitrate concentrations were highest for tributaries draining developed and agricultural sites, suggesting anthropogenic inputs. However, it was difficult to determine the source of nitrate because of the wide scatter in the nitrogen isotopic data, which for the most part did not correlate with land use. The only systematic trends in nitrogen isotopic values are found in waters collected in the summer. However, these data show that δ15N values of nitrate from agricultural sites were lower than δ15N values of nitrate from forested sites, a result that is inconsistent with previous studies. Our study suggests that strontium isotopes provide a better biomarker for tracking Atlantic salmon (Salmo salar) than nitrogen isotopes because unlike 15N/14N ratios, 87Sr/86Sr ratios are both predictable and seasonally constant in water.