Water Resources Research Act Program

Details for Project ID 2020WI310B

Fate of Groundwater Phosphorus from Septic Systems Near Lakes

Institute: Wisconsin
Year Established: 2020 Start Date: 2020-03-01 End Date: 2021-02-28
Total Federal Funds: $36,223 Total Non-Federal Funds: $19,923

Principal Investigators: James Hurley

Abstract: This project will examine the fate of phosphorus in the near-shore areas of three lakes. The goal is to develop a new approach for estimating phosphorus contributions from septic systems to lakes. The specific objectives are to identify where septic system-impacted groundwater is entering the lakebed and determine the phosphorus concentration in that groundwater, and then relate those concentrations to the ability of the soil to retain phosphorus. Previous research has not provided a tool to evaluate the impact of septic systems to a lake’s phosphorus load. As lakes become increasingly surrounded by conventional septic systems, it is important to be able to communicate the extent to which these systems will impact a lake’s long-term biological productivity. This project is designed to contribute to our understanding of phosphorus migration from septic systems to lakes by: 1) mapping groundwater geochemistry and phosphorus concentrations in the lakebed adjacent to septic systems; and, 2) relating those measurements to phosphorus retention within nearby soil profiles.Phosphorus removal in a conventional septic system depends on the amount of phosphorus retained by soils beneath the drainfield. In this research, we employ innovative septic tracing tools (e.g., small-scale temperature profiling, low-flow geochemical evaluation, and artificial sweetener analysis) to determine where groundwater impacted by septic systems is entering the lake. Phosphorus concentrations in this groundwater will be measured to determine if septic systems are increasing phosphorus load to the lake. By comparing our measured phosphorus concentrations to soil sorption properties, we will better evaluate how septic systems could alter lakes. Previous research suggests a limit to the amount of phosphorus the soil beneath a drainfield can retain. Interestingly, few studies have shown elevated phosphorus concentrations in lakebed groundwater impacted by septic systems. It is challenging to measure the impact of septic systems in lakebeds because of the dilution from converging groundwater flow paths and when there are areas in the lakebed with locally high phosphorus concentrations not associated with areas of high groundwater inflow. This research proposed seeks to overcome these sampling limitations by using high resolution geochemical sampling to reduce the influence of dilution in lakebeds where we have converging groundwater flow and artificial sweetener analysis to recognize the septic signature.This research will provide a way to determine where septic systems are important contributors to the external phosphorus load to a lake. Lake groups and county and state agency staff will find this information useful in their planning and lake management work. The results of this research will address several priorities of the Joint Solicitation. These include priorities of the University of Wisconsin System (e.g., the impact of land-use practices on groundwater quality and the interactions of groundwater and surface water); the Wisconsin DNR (e.g., occurrence of groundwater contaminants and the geochemical conditions affecting their mobility in groundwater recharge areas) and the DSPS On-Site Wastewater (e.g., protecting groundwater and surface water quality).