Groundwater Mounding and Minnesota ISTS Rules MPCA 2/21/06

What is GW mounding? Subp. 4. Groundwater mound. “Groundwater mound”, means the rise in height of the seasonally saturated soil or regional water table caused by the addition of sewage effluent from a SSTS into the soil. Subp. 4. Groundwater mound. “Groundwater mound”, means the rise in height of the seasonally saturated soil or regional water table caused by the addition of sewage effluent from a SSTS into the soil.

What is GW mounding? Simply, the rise in the groundwater when water is added by man. Simply, the rise in the groundwater when water is added by man.

What is GW mounding?

Much More Water Natural recharge to the groundwater ranges from 3 to 11 inches per year. Natural recharge to the groundwater ranges from 3 to 11 inches per year. The actual bottom loading to a SSTS (no inflated flow values) ranges from 6 to 30 feet per year. The actual bottom loading to a SSTS (no inflated flow values) ranges from 6 to 30 feet per year.

So What is the Problem? Reduce the unsaturated zone for pathogen removal Reduce the unsaturated zone for pathogen removal

So What is the Problem? Impede oxygen transfer needed to breakdown the biomat Impede oxygen transfer needed to breakdown the biomat oxygen

So What is the Problem? Breakout in downslope areas Breakout in downslope areas

So What is the Problem? Reduced transmissivity? Reduced transmissivity?

Mounding and Mounding and ISTS Mounding accounted for 7080 “cookbook” designs. Mounding accounted for 7080 “cookbook” designs. Three-foot separation distance required on all soil textures – balance between treatment and mounding. Three-foot separation distance required on all soil textures – balance between treatment and mounding.

Mounding and ISTS Low surface area High surface area Sandy SoilHeavy Soil

History Agency recognized mounding concerns since 1984 (HRSA document). Agency recognized mounding concerns since 1984 (HRSA document). With the greatly increased number of LSTS and MSTS, new concerns have been raised. With the greatly increased number of LSTS and MSTS, new concerns have been raised.

Concerns What is the relationship between linear loading rate and mounding? What is the relationship between linear loading rate and mounding?

Linear Loading Rate

Concerns Are GW mounding models appropriate for seasonally saturated soil layers with slow permeability? Are GW mounding models appropriate for seasonally saturated soil layers with slow permeability?

Model Applicability Regional AquiferPermeable Materials

Model Applicability Seasonally Saturated Soil Slowly Permeable Materials

Concerns Have the models proved accurate in the past? Have the models proved accurate in the past? When should, and to what detail should a mounding analysis be conducted. When should, and to what detail should a mounding analysis be conducted.

Concerns What type and what frequency of field measurements should be used? What type and what frequency of field measurements should be used? What operational monitoring should be undertaken. What operational monitoring should be undertaken.

Rules – MSTS (Chapter 7081) –Preliminary groundwater evaluation must be conducted for all MSTS –Evaluation = the size of the system, proposed loading rate, and system geometry must be determined –from the best available information: determine the general geology, shallow groundwater setting, regional groundwater setting, hydraulic conductivities determine the general geology, shallow groundwater setting, regional groundwater setting, hydraulic conductivities

Rules – MSTS (Chapter 7081) –The designer must consult with the local unit of government to determine whether the local unit of government will require a field or further groundwater investigation and, if so, the extent of the investigation.

Rules – MSTS (Chapter 7081) –The designer must consult with the local unit of government to determine if the local unit of government will require monitoring and, if so, the extent of the monitoring.

Rules – MSTS (Chapter 7081) –An approximation of the rise in groundwater from system operation must be determined –A narrative evaluation of the accuracy of the approximation must be provided.

SDS Permit - LSTS The direction of groundwater flow The direction of groundwater flow The depth to the static groundwater level The depth to the static groundwater level The rate of groundwater movement (hydraulic conductivity) The rate of groundwater movement (hydraulic conductivity)

SDS Permit - LSTS The hydraulic gradient The hydraulic gradient The specific yield The specific yield To provide geologic cross-sectional analysis of the site including the presence of deeper limiting layers, perched water tables, bedrock, karst, etc. To provide geologic cross-sectional analysis of the site including the presence of deeper limiting layers, perched water tables, bedrock, karst, etc.

SDS Permit - LSTS To characterize aquifers that may be impacted To characterize aquifers that may be impacted

SDS Permit - LSTS The items involve hydrogeologic field work operations. The items involve hydrogeologic field work operations. This field work involves the drilling of deep borings at the proposed site This field work involves the drilling of deep borings at the proposed site The borings are used to test for the direction of groundwater flow, the static water level, the hydraulic conductivity, the hydraulic gradient, and the specific yield. The borings are used to test for the direction of groundwater flow, the static water level, the hydraulic conductivity, the hydraulic gradient, and the specific yield.