Characterization of shallow groundwater quality in the Lower St. Johns River Basin: a case study
Characterization of groundwater quality allows the evaluation of groundwater pollution and provides information for better management of groundwater resources. This study characterized the shallow groundwater quality and its spatial and seasonal variations in the Lower St. Johns River Basin, Florida, USA, under agricultural, forest, wastewater, and residential land uses using field measurements and twodimensional kriging analysis. Comparison of the concentrations of groundwater quality constituents against the US EPA’s water quality criteria showed that the maximum nitrate/nitrite (NOx) and arsenic (As) concentrations exceeded the EPA’s drinking water standard limits, while the maximum Cl, SO4 2−, andMn concentrations exceeded the EPA’s national secondary drinking water regulations. In general, high kriging estimated groundwater NH4 + concentrations were found around the agricultural areas, while high kriging estimated groundwater NOx concentrations were observed in the residential areas with a high density of septic tank distribution. Our study further revealed that more areas were found with high estimated NOx concentrations in summer than in spring. This occurred partially because of more NOx leaching into the shallow groundwater due to the wetter summer and partially because of faster nitrification rate due to the higher temperature in summer. Large extent and high kriging estimated total phosphorus concentrations were found in the residential areas. Overall, the groundwater Na and Mg concentration distributions were relatively more even in summer than in spring. Higher kriging estimated groundwater As concentrations were found around the agricultural areas, which exceeded the EPA’s drinkingwater standard limit.Very small variations in groundwater dissolved organic carbon concentrations were observed between spring and summer. This study demonstrated that the concentrations of groundwater quality constituents varied from location to location, and impacts of land uses on groundwater quality variation were profound.