Land Use Influence of Characteristics of Groundwater Inputs to the Great Bay Estuary, New Hampshire

Responding to the concerns that have arisen regarding elevated nitrate concentrations of groundwater discharging to the Great Bay Estuary (NH), the relationship between land use and water chemistry was investigated by coupling GIS-based land use data with CFC-derived groundwater ages, boron isotopes, and major ion chemistry. Seven submarine groundwater discharge (SGD) sites were selected and then groundwater monitoring networks installed and sampled to examine the relationship between land use and groundwater quality at the discharge zones. Field activities were performed in the summer and fall of 2003. Estuarine water intrusion in groundwater samples compromised the analysis for major ion chemistry and boron isotopes. CFC-derived and lumped parameters-modeled groundwater ages in the study area averaged 23.2 years (±15.0 years). CFC analysis allowed correlating the observed nitrate concentrations at the SGD sites with the land use coverage of 1974 (for most of the sites) or 1962 (for SGD 58.4). Two types of correlation were made: one between the agricultural and residential land use with all the observed nitrate concentrations in the SGD source areas and the other with the nitrate concentrations between developed and undeveloped land uses. Both correlations (Kendall's Tau and Spearman's Rho for each correlating group) indicated the increase of residential land use of the last three decades as being correlated with the high nitrate-bearing groundwater discharging to the Great Bay (NH). The geochemical composition of the SGD water was also investigated by using simple mixing models that attempted to explain the water chemistry characteristics of the targeted SGD sites. Overburden groundwater comprises 75% to 95% of the groundwater discharging at the SGD sites. A significant correlation (Tau's, p=0.021) between nitrate-bearing groundwater and CFC-derived groundwater ages was detected supporting the hypothesis that high nitrate-bearing groundwater will be discharged to the Great Bay in the near future accounting for the increase of residential land use of 1990's. Continuous monitoring of SGD sites was suggested to be included as part of the periodic environmental quality monitoring activities of the Great Bay. Long-term step-wise sampling for groundwater dating is required to develop a stronger chronological evolution of groundwater nitrate inputs. Further research should concentrate on detailing the overburden water chemistry, flow paths, and nitrogen loading characteristics.