Changes in nearshore water quality from 1995 to 2014 and associated linkages to septic systems in Higgins Lake, MI

Executive Summary

This report presents a new water quality dataset for Higgins Lake, mostly collected in 2014, along with an analysis of these data relative to prior water quality data collected by the USGS, and human wastewater systems around the lake. Samples were collected at 21 sites in the near shore region of the lake, including some sites that overlapped USGS sampling sites collected in the late 1990s. Four sampling events were conducted throughout the summer and early fall of 2014. A single partial early-morning shoreline dissolved oxygen and specific conductivity survey was also conducted.

A primary product of this report is a new and more in-depth analysis of data collected by the USGS spanning 1995-2000 (select sites were also sampled in 2007), alongside a similar analysis of newly collected data from this study’s 2014 sampling. This analysis highlights trends in water quality parameters, specifically those related to a major transition in wastewater treatment on the lake: the installation of a community wastewater treatment plant in 2009 for Camp Curnalia in the NW corner of the north basin of the lake.

Four datasets are analyzed in detail: total phosphorus (TP), nitrate+nitrite (NO3+NO2), specific conductivity, and boron (B). No significant temporal variations in concentrations were detected within the 2014 sampling period, thereby providing no support to the hypothesis that water quality is negatively impacted near the busy July 4th holiday. Comparing 2014 values to those from the late 1990s dataset showed a significant increase in near-shore surface water TP, which is of particular concern as the average concentration now exceeds the 12 ug/L mesotrophic threshold, where significant ecological changes begin to occur in the nearshore region lake. Specific conductivity values have steadily risen, indicating increased pollutant loads to the lake.

Temporal trends and spatial patterns in these data all support the hypothesis that the Camp Curnalia wastewater treatment plant has substantially improved water quality in the adjacent nearshore area, particularly due to groundwater inputs. Near Camp Curnalia at paired USGS/MSU sites, TP concentrations in groundwater have greatly decreased, and NO3+NO2 concentrations dropped below detection limits. B, an indicator of septic system inputs, also exhibited a significant decline in concentration. In addition, specific conductivity in the Camp Curnalia area was the lowest in the partial shoreline survey.

Statistical modeling was used to relate sampled concentrations of these and other water quality constituents to measures of septic system density and groundwater influx.

These models support that groundwater is a significant source of nutrients to the lake. Groundwater flow velocity into the lake, measured using a point-based seepage velocimeter, was the most significant variable explaining concentration patterns, while combinations of hydraulic gradient (how much slope there is in the water table near the lake) and septic/parcel density were also important.

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Posted on June 12, 2017 in Events