Farming on Karst and Fractured Bedrock Aquifers: Challenges, Best Practices, and the Silurian Bedrock Performance Standard

Introduction

Karst and Fractured Bedrock aquifers, characterized by their unique and permeable geological formations, present significant challenges for agricultural activities. These landscapes, formed from soluble rocks such as limestone, dolomite, and gypsum, and fractures from the weight of ice, feature sinkholes, caves, crevices, and underground rivers, which facilitate the rapid movement of water and contaminants. This article explores the distinct challenges of farming on karst and fractured bedrock aquifers, focusing on the risks of diffuse pollution from cash cropping and point pollution from direct conduits, the impact on local communities, and the best management practices to mitigate these risks. By understanding and implementing these practices, farmers can protect water quality while maintaining agricultural productivity.

Understanding Karst and Fractured Bedrock Geology

Karst and Fractured Bedrock terrains are defined by their high permeability due to extensive networks of fractures, conduits, and cavities. While Karst specifically refers to soluble carbonates, fractures also stem from the weight of ice during the ice ages, and continuing fractures from frost-thaw cycles. Karst and Fractured Bedrock features enable rapid water flow, which can quickly transport contaminants from the surface to the groundwater. The thin soil layer, or overburden, in these areas provides minimal filtration, making karst aquifers highly susceptible to pollution.

Diffuse Pollution from Cash Cropping

Diffuse pollution, also known as non-point source pollution, refers to the widespread contamination that occurs over large areas, typically from agricultural activities. In karst regions, cash cropping can contribute significantly to diffuse pollution through the application of fertilizers, pesticides, and herbicides. These substances can easily leach into the groundwater, especially when heavy rainfall or irrigation causes runoff.

In Kemble (Grey County), drainage ditches along fields expose fractured carbonate bedrock. After being in pasture for decades, this field is now plowed every year. Soil remains bare during the wet season between November to end of June, often with ponding water.

Aerial view of sinkholes in a close-cropped field along the Niagara Escarpment. Credit: Door County Soil and Water Conservation Department

Point Pollution from Conduits

Point pollution, or point source pollution, originates from specific, identifiable sources such as manure storage facilities, septic systems, and direct discharge into sinkholes. In karst regions, conduits like sinkholes and disappearing streams can act as direct pathways for contaminants to enter the groundwater system, bypassing natural filtration processes. This type of pollution is particularly concerning because it can introduce high concentrations of pollutants into the aquifer, posing immediate threats to water quality.

Cross-sectional diagrams for six types of sinkholes defined by Waltham et al. (2005) and b) illustration of sinkholes in Florida (modified from Rupert and Spencer 2004). Source: https://link.springer.com/article/10.1007/s10040-015-1333-3/figures/1

Field simplification and land consolidation despite Karst

When large cash cropping or feedlot operations integrate more land into their ownership, the first step is usually land simplification. Removal of hedgerows and rock piles, leveling of topography, filling in sinkholes. In most jurisdictions, there is no separate legal category for sinkholes – they are just treated as a small depression. Yet, if they are filled in farmers create “buried sinkholes” that loose materials into the ground and create hollows. These hollows eventually collapse, there are many cases when they swallowed even large machinery. These accidents become insurance cases. The fill material enters the aquifer and may considerably alter the Karst hydrology, by clogging conduits and re-routing waterflow. The loose fill also injects nutrients and other contaminants into the aquifer system. The original sinkhole depressions usually had a shallow soil cover (like the “solution sinkhole” in the figure above) that is held in place by dense plant roots. This wild vegetation thrives due to access to water, regular inflow of nutrients, and some wind protection, and is an important part of a healthy water cycle.

Impact on Citizens

Contaminated groundwater has severe implications for the health and well-being of local communities. Residents relying on private wells for drinking water are particularly vulnerable to the effects of pollution. Contaminants such as nitrates, bacteria, and pesticides can cause a range of health issues, including gastrointestinal illnesses, methemoglobinemia (blue baby syndrome), and long-term risks such as cancer. Beyond health impacts, water contamination can also decrease property values, leading to economic hardships for affected families.

Best Management Practices (BMPs) for Farming on Karst Aquifers

To mitigate the risks of diffuse and point pollution in karst regions, farmers can implement several best management practices:

1. Permanent Pastures: Maintaining permanent pastures that are never ploughed can significantly protect karst aquifers. These pastures prevent soil disturbance, reduce erosion, and provide a continuous cover that minimizes the risk of contaminants reaching the groundwater.
2. Soil Conservation: Practices such as using cover crops, reducing tillage, and maintaining grass buffer strips help reduce soil erosion and nutrient losses. These measures are particularly effective in preventing diffuse pollution from agricultural runoff.
3. Precision Agriculture: Utilizing precision agriculture techniques allows farmers to apply nutrients and pesticides more efficiently, reducing the risk of over-application and subsequent leaching into groundwater.
4. Nutrient Management Plans: Developing and adhering to nutrient management plans ensures that fertilizers and manure are applied at appropriate rates and times, minimizing the potential for contamination.
5. Proper Manure Management: Avoiding manure spreading on excessively dry clay soils and ensuring that manure is applied in smaller volumes can prevent direct contamination of groundwater. Manure should also be managed to avoid runoff into sinkholes or other karst features.
6. Monitoring and Testing: Regular testing of soil and water quality helps identify contamination risks early. Farmers can then adjust their practices to mitigate these risks effectively.
7. Buffer Zones: Establishing buffer zones around sinkholes, disappearing streams, and other karst features can prevent direct contamination from agricultural activities.

Concrete Prohibitions to Protect Karst Aquifers

To further protect karst aquifers, the following prohibitions should be enforced:

• No Manure Spreading on Thin Soils: Prohibit the spreading of manure on soils less than 2 feet deep to prevent direct leaching into groundwater.
• Distance Restrictions for Manure Storage: Manure storage facilities should be located at least 150 feet away from wells and direct conduits to groundwater.
• Limitations on Fertilizer Application: Restrict fertilizer application near sinkholes and other karst features to prevent direct contamination.
• Proper Septic System Maintenance: Ensure that septic systems are regularly inspected and maintained to prevent leaks and failures that can contaminate groundwater.

Case Study: Kewaunee County, Wisconsin

The severe water contamination crisis in Kewaunee County, Wisconsin, provides a stark warning about the risks of farming on karst aquifers without proper management and regulation. In Kewaunee County, extensive agricultural activities, particularly from concentrated animal feeding operations (CAFOs), led to widespread groundwater contamination. Testing revealed that over 60% of private wells were contaminated with fecal microbes, primarily from bovine sources.

The contamination had dire health impacts on the local population, causing numerous cases of gastrointestinal illness and other health issues. The crisis highlighted the need for stringent regulations and proactive measures to protect water quality in karst regions.

Regulatory Response and Community Engagement

In response to the crisis, Kewaunee County implemented several regulatory changes and engaged the community in efforts to improve water quality [1]. The establishment of a bipartisan Water Quality Task Force led to the development of strategies aimed at addressing the contamination. These included better soil mapping, improved management of agricultural runoff, and changes to well and septic system construction.

The legal battle known as Clean Water vs. DNR further emphasized the community’s struggle for clean water. The Wisconsin Supreme Court ruling in favor of the residents led to stricter regulations on CAFOs, including limits on herd size and requirements for groundwater monitoring.

The Silurian Bedrock Performance Standards

The core piece of legislation that came out of this debacle was the Silurian Bedrock Performance Standards that the Wisconsin Department of Natural Resources (DNR) developed. It addresses the unique challenges of managing agricultural practices on sensitive bedrock areas, particularly those with shallow soils overlying Silurian dolomite. These standards are outlined in NR 151.075 and aim to protect water quality by regulating the application of manure and other agricultural activities in regions where the bedrock is highly susceptible to contamination. Details of this standard were under legal review of the Supreme Court challenge, and its scientific robustness was confirmed.

Key Provisions of the Silurian Bedrock Performance Standards

1. Manure Application Restrictions:
   • Liquid Manure: The application of liquid manure is strictly controlled based on the depth of soil to the bedrock. For instance:
     • For soils with 2 to 3 feet depth to bedrock, the maximum annual application rate is 6,750 gallons per acre.
     • For soils with 5 to 20 feet depth to bedrock, the maximum weekly application rate can be up to 27,000 gallons per acre, depending on soil texture.
   • Solid Manure: Solid manure must be incorporated into the soil within 72 hours of application and at specific rates to minimize contamination risks.
2. Pre-Tillage Requirements:
   • Fields must be pre-tilled to a specified depth unless the land meets long-term no-till criteria or has a perennial or established crop. This helps in reducing surface runoff and potential contamination.
3. Pathogen Reduction:
   • Manure must be treated to reduce pathogen levels significantly. For example, liquid manure should be treated to achieve a fecal coliform bacteria density of less than 500,000 colony-forming units per 100 milliliters.
4. Nutrient Management Compliance:
   • All manure applications must comply with the University of Wisconsin’s nutrient application guidelines (A2809). This ensures that nutrient applications are within agronomic rates and do not exceed the soil’s ability to absorb and utilize these nutrients, thereby preventing excess runoff and groundwater contamination.
5. Verification of Soil Depth:
   • It is mandatory to verify soil depth to bedrock through in-field assessments or using existing maps and data to ensure compliance with the performance standards.

Purpose and Impact

The purpose of these standards is to mitigate the risk of groundwater contamination in areas where the bedrock is close to the surface and easily fractured. By regulating the application of manure and implementing stringent nutrient management practices, the DNR aims to protect water quality in these sensitive areas, which are prone to rapid and extensive contamination due to their geological characteristics.

The Silurian Bedrock Performance Standards represent a proactive approach to addressing environmental concerns related to agricultural practices in vulnerable regions, ensuring sustainable farming while safeguarding public health and the environment. While it had many successes, there are also unresolved issues that are currently debated in the communities. Most importantly, Nitrate levels continue to be high in groundwater. Especially if the overburden material is permeable, then even 100 feet of sandy soil are insufficient to protect groundwater quality. Discussions are ongoing, and the stakeholder dialogue exemplifies the proactive management of water quality issues.

Conclusion

Farming on karst aquifers presents unique challenges due to the high permeability and rapid water flow in these regions. The risks of diffuse pollution from cash cropping and point pollution from direct conduits require careful management and adherence to best practices to protect water quality. The severe contamination crisis in Kewaunee County underscores the importance of proactive measures, community engagement, and stringent regulations to safeguard public health and prevent similar tragedies in other regions.

By implementing best management practices, maintaining permanent pastures, and enforcing concrete prohibitions, farmers can effectively manage agricultural activities in karst regions and protect valuable water resources. For more detailed guidance on best management practices for farming on karst and Silurian soils, visit the University of Wisconsin Extension website.

References

[1] Manure spills could give Kewaunee County’s first test of new ordinance.  Door County Daily News, 2018.

[2] Kewaunee County Ordinance Chapter 39 – Agricultural Performance Standard, which includes the Silurian Bedrock Performance Standard 39.075. https://www.kewauneeco.org/i/f/files/Land%20%26%20Water%20Conservation/Chapter%2039.pdf

[3] University of Wisconsin Extension website