4.1 Groundwater Vulnerability, Wisconsin Karst and Central Sands Hydrogeology, and ATCP 30

Groundwater Vulnerability, Wisconsin Karst and Central Sands Hydrogeology, and ATCP 30

Groundwater is one of Wisconsin's most vital natural resources, supplying drinking water to more than 70% of the state's residents and virtually 100% of rural farm families. Because groundwater resides beneath the soil surface in saturated geological formations known as aquifers, preventing pesticide contamination is a primary statutory responsibility of every certified commercial and private applicator. Once a pesticide active ingredient or its breakdown metabolites infiltrate an aquifer, remediation is extraordinarily difficult, economically prohibitive, and ecologically damaging. Natural attenuation in dark, anaerobic, cold underground environments can take decades or centuries.

Under Wisconsin Statutes Chapter 94 and Wisconsin Administrative Code Chapters ATCP 29 and ATCP 30, the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) actively monitors and regulates pesticide applications to safeguard groundwater quality. Applicators must understand the physical characteristics of pesticides, the hydrogeological vulnerabilities of Wisconsin's soils and bedrock, and the strict legal limitations governing vulnerable agricultural landscapes.

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1. Wisconsin Hydrogeology: Two Critical Leaching Landscapes

Wisconsin's geological history, shaped by repeated glacial advances and retreats, has produced diverse soil profiles and bedrock formations. Two distinct geographic regions represent extreme groundwater contamination hazards: the Central Sands and the Eastern Karst Region.

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|                  WISCONSIN'S TWO CRITICAL LEACHING LANDSCAPES               |
|                                                                             |
|   [CENTRAL SANDS OUTWASH PLAIN]              [EASTERN / DOOR COUNTY KARST]  |
|   - Coarse sand / loamy sand soils           - Thin glacial topsoil (< 3 ft)|
|   - Minimal organic matter (< 1.5%)          - Fractured Silurian dolomite  |
|   - Rapid hydraulic conductivity             - Sinkholes & solution crevices|
|   - Shallow water table (5 to 15 ft)         - Disappearing surface streams |
|   - High irrigation driving percolation      - DIRECT conduit to aquifers   |
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1. The Central Sands Region

The Central Sands region (encompassing Portage, Waushara, Adams, Wood, and Juneau counties) is an extensive glacial outwash plain characterized by unique hydrogeological vulnerabilities:

• Coarse Soil Texture & Low Organic Carbon: The soils consist predominantly of coarse sands and loamy sands with very low organic matter content (frequently less than 1.0% to 1.5%). Organic matter and clay particles provide the primary electrostatic binding sites that retain pesticide molecules; their absence leaves pesticides unadsorbed in soil pore water.
• High Hydraulic Conductivity: Sandy soils possess large macropores that allow rainwater and irrigation water to percolate downward at rapid rates (often several inches per hour), giving soil microbes little time to degrade chemical compounds.
• Shallow Unconfined Aquifers: The depth from the surface to the groundwater table in the Central Sands is frequently only 5 to 15 feet. Under intensive center-pivot irrigation, mobile pesticides migrate from the crop root zone into drinking water aquifers in a matter of days or weeks.

2. Eastern Wisconsin and Door County Karst Topography

Karst topography dominates the eastern ridge of Wisconsin, extending through Door, Kewaunee, Brown, Manitowoc, and Calumet counties. This landscape is underlain by fractured Silurian dolomite (limestone) bedrock:

• Dissolution Features: Over thousands of years, slightly acidic groundwater dissolved the carbonate rock along natural joints and bedding planes, creating an underground network of enlarged fractures, caves, solution crevices, and sinkholes.
• Disappearing Streams and Direct Conduits: Surface runoff, field drainage, and intermittent streams often drain directly into sinkholes or bedrock fissures, bypassing all natural soil filtration. A chemical spill or surface runoff event in a karst landscape enters the deep groundwater aquifer within minutes as unfiltered surface water.
• Thin Topsoil Mantle: In many areas of Door and Kewaunee counties, the protective soil layer overlying fractured bedrock is less than 18 to 36 inches thick (and in some areas less than 12 inches). Thin soils lack the physical depth and microbial biomass required to filter or biochemically degrade pesticides before they hit bedrock fissures.

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2. Chemical Fate Parameters: Mobility and Leaching Metrics

Whether a pesticide reaches groundwater depends on the interplay between environmental site factors (soil, rainfall, depth to water) and the intrinsic physicochemical properties of the pesticide active ingredient.

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|                   PHYSICOCHEMICAL PESTICIDE MOBILITY DRIVERS                |
|                                                                             |
|   [WATER SOLUBILITY (Sw)]   ---> High solubility (> 30 mg/L) dissolves      |
|                                  freely in downward percolating water.      |
|   [ADSORPTION (Koc)]        ---> Low Koc (< 300 mL/g) indicates weak        |
|                                  binding to soil organic matter.            |
|   [PERSISTENCE (DT50)]      ---> Long half-life (> 30 days) allows chemical |
|                                  to survive long enough to reach water table|
|                                                                             |
|   * HIGH LEACHER PROFILE = High Sw + Low Koc + Long DT50 (High Persistence) *
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1. Water Solubility ($S_w$)

Water solubility measures the maximum mass of a pesticide active ingredient that can dissolve in a given volume of water at a standardized temperature (typically 20°C or 25°C), expressed in milligrams per liter (mg/L) or parts per million (ppm).

• Highly soluble chemicals ($S_w > 30\text{ mg/L}$) dissolve readily in soil moisture and move freely with downward gravitational water flow.
• Pesticides with low solubility ($S_w < 1\text{ mg/L}$) tend to precipitate or remain bound to soil surfaces.

2. Soil Organic Carbon Adsorption Coefficient ($K_{oc}$)

The $K_{oc}$ value quantifies the affinity of a pesticide molecule to bind (adsorb) to organic carbon in soil particles, measured in milliliters per gram (mL/g).

• High Mobility ($K_{oc} < 300\text{ to } 500\text{ mL/g}$): The pesticide binds weakly to soil particles and remains primarily dissolved in soil water, presenting a severe leaching hazard (e.g., atrazine $K_{oc} \approx 100\text{ mL/g}$; clopyralid $K_{oc} \approx 6\text{ mL/g}$).
• Moderate Mobility ($K_{oc} = 500\text{ to } 1,000\text{ mL/g}$): Moderate sorption; intermediate leaching potential.
• Immobile ($K_{oc} > 1,000\text{ to } 2,000+\text{ mL/g}$): The chemical binds tightly to organic matter and clay plates, resisting downward leaching (e.g., glyphosate $K_{oc} > 24,000\text{ mL/g}$; pyrethroid insecticides $K_{oc} > 100,000\text{ mL/g}$). Note that tightly bound chemicals can still move into surface waters attached to eroded sediment particles during runoff events.

3. Soil Field Dissipation Half-Life ($DT_{50}$ / Persistence)

The $DT_{50}$ (Dissipation Time 50%) is the time in days required for half of the applied pesticide mass to break down through microbial degradation, chemical hydrolysis, and photodegradation under field conditions.

• Non-Persistent: $DT_{50} < 30\text{ days}$ (degrades before significant downward migration occurs).
• Moderately Persistent: $DT_{50} = 30\text{ to } 100\text{ days}$.
• Persistent: $DT_{50} > 100\text{ days}$ (survives through seasons, presenting cumulative leaching risks).

The Groundwater Ubiquity Score (GUS Index)

Toxicologists combine persistence and adsorption into the GUS Index to classify leaching hazard:

$\text{GUS} = \log_{10}(DT_{50}) \times [4 - \log_{10}(K_{oc})]$

• GUS $> 2.8$: Extremely High Leacher (Strict management required; prone to RUP groundwater status).
• GUS $1.8\text{ to } 2.8$: Transition Leacher (Site-specific precautions mandatory).
• GUS $< 1.8$: Non-Leacher (Negligible groundwater hazard under standard agronomic practices).

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3. Wisconsin Administrative Code ATCP 30: The Atrazine Rule

Wisconsin Administrative Code Chapter ATCP 30 (Pesticide Product Restrictions) was promulgated by DATCP to address widespread contamination of Wisconsin drinking water wells by the triazine herbicide atrazine and its chlorinated metabolites (deethylatrazine, deisopropylatrazine, and diaminoatrazine).

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|                      WISCONSIN ATCP 30 ATRAZINE REGULATION                  |
|                                                                             |
|   [GROUNDWATER ENFORCEMENT STANDARD (ES)] = 3.0 ppb (µg/L) Total Chlorinated|
|   [PREVENTIVE ACTION LIMIT (PAL)]         = 0.3 ppb (µg/L) Residues         |
|                                                                             |
|   [STATEWIDE RATE CAPS]                                                     |
|   - Coarse Soils:   Max 0.75 lb active ingredient / acre / calendar year    |
|   - Medium / Fine:  Max 1.0 to 1.5 lb active ingredient / acre / year       |
|                                                                             |
|   [ATRAZINE PROHIBITION AREAS (PAs)]                                        |
|   - Over 100 designated geographic zones (> 1.2 million acres)               |
|   - Total ban: NO person may apply, mix, or load atrazine for any purpose.  |
|                                                                             |
|   [RECORDKEEPING MANDATE] = Retain all application records for 3 YEARS      |
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1. Groundwater Quality Standards for Atrazine

Wisconsin evaluates groundwater contamination using two statutory benchmark thresholds under Wis. Admin. Code Chapter NR 140:

• Preventive Action Limit (PAL): Set at $0.3\text{ ppb}$ (micrograms per liter, $\mu\text{g/L}$) total atrazine and chlorinated metabolites. Exceeding the PAL triggers mandatory regulatory monitoring and corrective agronomic adjustments.
• Enforcement Standard (ES): Set at $3.0\text{ ppb}$ ($\mu\text{g/L}$) total atrazine and chlorinated metabolites. This is the legal health threshold. If well testing reveals concentrations exceeding 3.0 ppb, DATCP is statutorily mandated to establish an Atrazine Prohibition Area.

2. Statewide Annual Application Rate Limits

Under ATCP 30, atrazine application rates are strictly capped based on soil texture and prior-year application history:

[!IMPORTANT] Coarse Soil Rate Cap: On coarse-textured soils (common throughout the Central Sands and river valleys), the maximum legal rate is $0.75\text{ lb}$ active ingredient per acre per calendar year, regardless of whether atrazine was applied the previous season.

3. Atrazine Prohibition Areas (PAs)

When DATCP or Department of Natural Resources (DNR) groundwater testing reveals atrazine concentrations at or above the 3.0 ppb Enforcement Standard, DATCP establishes an Atrazine Prohibition Area (PA) through administrative rulemaking:

• Scope: Wisconsin has established over 100 designated Atrazine Prohibition Areas, encompassing more than 1.2 million agricultural acres across the state (including large portions of Dane, Columbia, Sauk, Lafayette, Grant, and Outagamie counties).
• Prohibition Mandate: Within a designated PA, no person may apply, mix, or load any atrazine product for any agricultural or non-agricultural purpose under any circumstance. Applying atrazine in a prohibition area is a major state statutory violation.
• Applicator Responsibility: Applicators must consult DATCP's official county Atrazine Prohibition Area maps before recommending, purchasing, or applying any herbicide premix containing atrazine.

4. Mandatory 3-Year Record Retention

While general pesticide records under ATCP 29 must be retained for 2 years, ATCP 30 mandates that all records of atrazine applications must be retained for at least 3 years. Records must document the exact field location (quarter-section, section, township, range), soil texture, application rate in pounds of active ingredient per acre, and whether the field is in a prohibition area.

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4. Point Source vs. Non-Point Source Contamination & Well Setbacks

Groundwater contamination occurs through two distinct pathways, each requiring specialized operational prevention strategies.

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|               CONTAMINATION PATHWAYS & PREVENTION CONTROLS                  |
|                                                                             |
|   [POINT SOURCE CONTAMINATION]               [NON-POINT SOURCE LEACHING]    |
|   - Concentrated spills at mixing pads       - Field-wide broadcast sprays  |
|   - Unrinsed container dumping               - Runoff across acreages       |
|   - Back-siphoning into wellhead             - Percolation through sands    |
|   - Prevent: 100-ft Setback, Pad, Air Gap    - Prevent: Rate caps, IPM, Koc |
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1. Point Source Contamination

Point source contamination originates from a specific, identifiable, and localized site where concentrated pesticides are handled, loaded, mixed, or stored. Point sources represent the most severe acute threats to drinking water wells because pure chemical concentrate enters the ground in high concentrations:

• Spills and Leaks: Concentrated product spills during mixing/loading on unprotected gravel or dirt driveways.
• Equipment Washdown: Discharging tank rinsate or washing sprayer booms in field corners near well casings.
• Back-Siphoning (The Ultimate Point Source Hazard): Occurs when a filling hose is submerged below the liquid level in a spray tank. If water supply pressure drops suddenly (e.g., pump failure, power outage, or nearby hydrant draw), the spray mixture is sucked backward through the hose directly into the well or municipal plumbing.

[!CAUTION] Back-Siphoning Prevention Standards: Applicators must prevent backflow by implementing one of two mandatory mechanical safeguards:

1. Air Gap: Maintain a physical, unobstructed vertical air space between the end of the water supply pipe/hose and the flood rim of the spray tank equal to at least twice the inside diameter of the supply hose (minimum 1-inch air gap).
2. Backflow Preventer: Install an approved, functioning Reduced Pressure Zone (RPZ) backflow preventer or vacuum breaker on the water supply line.

2. Non-Point Source Contamination

Non-point source contamination results from diffuse, widespread movement of pesticides over extensive land areas following standard agronomic field applications. Examples include uniform leaching through permeable Central Sands fields following heavy rain, or broadsheet runoff across rolling hills into receiving waterways. Non-point contamination is mitigated by reducing overall chemical loading, selecting high-$K_{oc}$ products, and observing strict label application rate caps.

3. Well Setback Standards (ATCP 29.45 & NR 811/812)

To eliminate point source wellhead contamination, Wisconsin law enforces strict physical separation distances:

• The 100-Foot Mixing/Loading Setback (ATCP 29.45): No person may mix, load, or clean pesticide equipment within 100 feet of any potable well (private residential, commercial, or agricultural well) or surface water body UNLESS the operation occurs on an approved DATCP secondary containment pad.
• Containment Pad Engineering: The pad must be constructed of portland cement concrete or another nonabsorbent, liquid-tight material, designed to catch and contain all reasonably foreseeable spills, and — for liquid pesticides — served by a pump plumbed to a storage container that keeps at least 200 gallons of unused capacity available at all times (ATCP 29.45(4)(a)2).