Dakota County Releases New Groundwater Report

by Vanessa Demuth, Dakota County

Introduction

More than 90 percent of Dakota County residents rely on groundwater for their water supply, whether it comes from a public water supplier or from a private drinking water well. Dakota County residents expect an abundance of clean water for drinking; however, the three principal drinking water aquifers in the county – the unconsolidated sediments, the Prairie du Chien and the Jordan Aquifers (in order of depth) – are vulnerable to contamination and have been impacted by contaminants from both human activities and naturally-occurring, geologically sourced chemicals.

The Dakota County Ambient Groundwater Quality Study began in 1999 to establish a baseline of water quality conditions to which ongoing conditions could be compared over a 20-year period. The study evaluated groundwater conditions in wells across the County that were selected to represent all three principal drinking water aquifers, in a variety of land-use, soil, and geological settings. Sampling a total of 77 private wells multiple times enabled the County to monitor long-term trends in groundwater contamination from human (anthropogenic) sources and activities, such as nitrogen fertilizer and herbicides commonly used on corn crops, and chloride from salt applied to roads for deicing, potassium fertilizer (potash) and water softeners. The study also enabled the County to identify where the groundwater contains geologically sourced contaminants, such as manganese or arsenic, and to monitor the County’s groundwater for industrial chemicals and Contaminants of Emerging Concern, such as per- and polyfluoroalkyl substances (aka PFCs, PFAS, or 3M chemicals), medications, personal care products, household products such as cleaners, and ingredients related to manufacturing.

This report summarizes the data collected from 1999 through 2019 and includes data from other sampling events and studies that occurred in the County, including the Hastings Area Nitrate Study (HANS), the MN Department of Agriculture’s (MDA) Township Testing Program, the Wells and Increased Infant Sensitivity and Exposure Study (WIISE), and the Burnsville, Greenvale and Lakeville Community- Focused Private Well Sampling.

In Minnesota, the Dakota County Ambient Groundwater Quality Study is a unique reference resource. While other studies may provide snapshots of water quality at a single point in time, the Ambient Study provides a chronicle of private well drinking water quality over 20 years. The Dakota County Ambient Study includes one of the most comprehensive sets of data about agricultural herbicides in private wells in the State.

Key Findings

1. Contaminant levels exceed health guidelines for both naturally occurring and anthropogenic chemicals in the principal aquifers used for drinking water. Over the course of the study, sixty-two percent of the sampled wells contained concentrations of at least one chemical contaminant exceeding current Minnesota Department of Health (MDH) drinking water guidelines. Drinking water guidelines do not exist for every contaminant detected. Some wells had multiple chemicals detected that exceed a guideline. Anthropogenic compounds are largely detected in the central and eastern portion of the County where row crop agriculture is the dominant land use. For example, 38 percent of the wells in the study exceed the drinking water guidelines for nitrate as nitrogen (nitrate) or the discontinued herbicide cyanazine. Cyanazine was a weed killer used primarily on corn and soybean crops. The percent of wells that exceed an established drinking water guideline for a chemical are:

Chemical	Percent of Wells Exceeding Drinking Water Guideline At Least Once Between 1999-2019
Manganese	34 percent of wells sampled exceed the guideline of 0.100 mg/L (milligrams per liter or parts per million)
Nitrate	31 percent exceed the guideline of 10 mg/L
Cyanazine breakdown products	22 percent exceed the guideline of 1 ug/L (micrograms per liter or parts per billion
Gross Alpha	3 percent exceed the guideline of 15 pCi/L (picocuries per liter)

2. The occurrence and concentrations of anthropogenic contaminants in groundwater reflect land use and the depth of the specific well.
3. Nitrate is the most commonly detected anthropogenic contaminant exceeding the drinking water guideline.
4. There are more wells with an upward trend for nitrate than a downward trend.
5. In the central and eastern portions of Dakota County where row crop agriculture is the dominant land use, average nitrate levels of study wells exceed 10 mg/L and are increasing.
6. The breakdown products of herbicides commonly applied to corn and soybean crops are the most frequently detected pesticides and are found in more than 70 percent of the wells tested. Most herbicide concentrations are below the applicable drinking water guidelines except for the herbicide cyanazine.
7. The production and use of the herbicide cyanazine, common brand name Bladex, was discontinued in 2002; its breakdown products are still detected in 2019, in Ambient Study wells at levels that exceed the drinking water guideline.
8. Herbicide compound detections generally occur as mixtures with each other and with nitrate, and higher levels of nitrate are accompanied by an increase in the number of herbicide compounds.
9. Pesticides are detected in municipal wells in both the 2005-2006 and 2019 sample events.
10. Manganese, a naturally occuring, geologically sourced contaminant, was detected above the drinking water guideline in 34 percent of the wells.
11. Arsenic, another naturally occurring, geologically sourced contaminant, was detected in 39 percent of the wells, none over the drinking water guideline.
12. All three primary drinking water aquifers and well casing depth categories are susceptible to contamination.
13. Anthropogenic contaminants of emerging concern (CECs) are widespread at low levels.
14. The age of well water and vertical recharge rates can be estimated using herbicide, nitrate and chloride results over time.

Conclusions

The Ambient Study found that both anthropogenic and natural, geologically sourced contaminants are widespread in the drinking water aquifers: 62 percent of the wells tested exceed the drinking water guideline for one or more contaminants. Anthropogenic contaminants are persistent and moving deeper into the aquifers over time; with these contaminants, land use and well casing depth are the most important factors. By contrast, naturally occurring, geologically sourced contaminants—in particular, manganese—are widespread but difficult to predict.

Based on the County’s groundwater conditions, drilling a deeper well to find “cleaner” water is a poor long-term strategy for a private well owner: anthropogenic contaminants like nitrate, herbicides, or chloride are migrating deeper, so a deeper well will be a temporary fix, and elevated manganese or other geologically sourced contaminants may be found at any well depth. Better options are to prevent groundwater contamination wherever possible and to use appropriate water treatment where the groundwater is already contaminated.

While land use improvements limiting surface pollution sources may take decades to be fully effective, the sooner they are started, the sooner improvements will be seen. Land use changes implemented today to improve water quality will affect shallow wells first and deeper wells more slowly.

The Ambient Study has proved to be a useful framework to have in place for surveying the County’s drinking water aquifers when CECs have been identified. Because of the existing Ambient Study well network, the County was able to conduct widespread sampling for PFAS, organic wastewater components, and pharmaceuticals when these became concerns. Vigilance and awareness are called for as new chemicals are introduced to the environment and new laboratory water testing capabilities make it feasible to detect more contaminants at very low levels.

Whether the contaminants are anthropogenic, naturally occurring, or newly identified, the Ambient Study provides a solid basis for well owner education and outreach, and a baseline of groundwater data for comparison as groundwater conditions change in the future.

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