Identification of naturally-occurring fluoride and selected metals in northwest Wisconsin groundwater

Abstract

Northwest Wisconsin, encompassing the 11 counties of Ashland, Barron, Bayfield, Burnett, Douglas, Iron, Polk, Price, Rusk, Sawyer, and Washburn, has a large proportion of the population who obtain their drinking water from groundwater. There are over 56,000 public and private wells within this region of the state, however, the region is under-studied in terms of groundwater quality relative to other areas in Wisconsin. Fluoride and the metals iron, manganese, aluminum, arsenic, and lead, if present at high enough concentrations in groundwater, could impact public health. Little is known regarding baseline concentrations of these potential contaminants in northwestern Wisconsin aquifers. The objectives of this study, known as the Northwest Wisconsin Groundwater Monitoring Project, were to monitor naturally-occurring fluoride concentrations in groundwater samples collected from 11 counties in northwestern Wisconsin, determine concentrations of iron, manganese, aluminum, arsenic, and lead in a subset of these samples, foster public awareness among residents of the region about the need for regular private well testing, and increase the publicly-available groundwater data within the region. This monitoring study used a random sample design, and targeted 704 fluoride samples and 115 metals samples total. All of the samples were collected by volunteers who were recruited to participate in the project through a combination of social media postings, press releases, and outreach events. Sample kits containing sample bottles, instructions for collection, sample collection form, and a pre-paid shipping label were assembled and delivered to each of 26 distribution sites located throughout the 11-county study area. The data from 450 of 704 fluoride and 80 of 115 metals samples were reported. Samples were collected from all 11 counties in the study area, and were largely collected from drilled wells constructed 26 – 50 years ago, with a depth of 51 – 100 feet. None of the samples collected and analyzed for fluoride exceeded the Wisconsin Groundwater Quality Enforcement Standard (ES) of 4.0 mg/L and less than 1% exceeded the Preventative Action Limit (PAL) of 0.8 mg/L (Wis. Adm. Code Ch. NR 140). Iron concentrations were high in a number of samples, with 20% of samples collected exceeding the 0.3 mg/L ES. In 5% of samples collected, the ES for manganese, 300 µg/L, was exceeded. Overall, aluminum concentrations were low throughout the study area, but one sample exceeded the ES of 200 µg/L. The maximum arsenic concentration measured in this study was 10.0 µg/L, which is the state ES. However, 38.8% of samples had an arsenic concentration at or above the PAL (1 µg/L). There was a single sample that exceeded the ES for lead (15 µg/L) and 11.3% of samples had a lead concentration at or above the 1.5 µg/L PAL. To the authors’ knowledge, this study represents the first baseline groundwater monitoring effort encompassing the northwest Wisconsin region. With the exception of fluoride and arsenic, the ES was exceeded for all parameters measured in this study in at least one sample collected within the study area. This study utilized a random sample design, and the next logical step is to conduct targeted sampling of areas with contaminant-level metals concentrations in groundwater samples.