Simulating long term nitrate-N contamination processes in the Woodville Karst Plain using CFPv2 with UMT3D

Abstract

• This is an initial attempt of transport simulation by CFPv2 with UMT3D over regional scales. • Nitrate-N from two sprayfields is transported rapidly through karst conduit networks in the WKP. • CFPv2 and UMT3D simulated non-laminar flow and transport in explicit conduit networks. A research version of CFP (Conduit Flow Process) code, CFPv2, is applied with UMT3D to simulate long term (1966–2018) nitrate-N contamination transport processes in the Woodville Karst Plain (WKP), northern Florida, where karst conduit networks are well developed. Groundwater flow in the WKP limestone porous matrix is simulated using Darcy’s law, and non-laminar flow within conduits is described by Darcy-Weisbach equation. Nitrate-N conduit transport and advective exchanges of groundwater and nitrate-N between conduits and limestone matrix are calculated by CFPv2 and UMT3D, instead of MODFLOW and MT3DMS since Reynolds numbers for flows in conduits are over the criteria of laminar flow. The developed numerical model is calibrated by field observations and then applied to simulate nitrate-N transport in the WKP. The numerical simulations verify the theories that two sprayfields near the City of Tallahassee and septic tanks in the rural area are major nitrate-N point sources within the WKP. High nitrate-N concentrations occur near Lost Creek Sink, and conduits of Wakulla Spring and Spring Creek Springs where aquifer discharge groundwater. Conduit networks control nitrate-N transport and regional contaminant distributions in the WKP, as nitrate-N is transported through conduits rapidly and spread over large areas.