Quantification of Phosphorus Transport from a Karstic Agricultural Watershed to Emerging Spring Water

The degree to which waters in a given watershed will be affected by nutrient export can be defined as that watershed’s nutrient vulnerability. This study applied concepts of specific phosphorus (P) vulnerability to develop intrinsic groundwater vulnerability risk assessments in a 32 km2 karst watershed (spring zone of contribution) in a relatively intensive agricultural landscape. To explain why emergent spring water was below an ecological impairment threshold, concepts of P attenuation potential were investigated along the nutrient transfer continuum based on soil P buffering, depth to bedrock, and retention within the aquifer. Surface karst features, such as enclosed depressions, were reclassified based on P attenuation potential in soil at the base. New techniques of high temporal resolution monitoring of P loads in the emergent spring made it possible to estimate P transfer pathways and retention within the aquifer and indicated small–medium fissure flows to be the dominant pathway, delivering 52–90% of P loads during storm events. Annual total P delivery to the main emerging spring was 92.7 and 138.4 kg total P (and 52.4 and 91.3 kg as total reactive P) for two monitored years, respectively. A revised groundwater vulnerability assessment was used to produce a specific P vulnerability map that used the soil and hydrogeological P buffering potential of the watershed as key assumptions in moderating P export to the emergent spring. Using this map and soil P data, the definition of critical source areas in karst landscapes was demonstrated.