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WQ656, Assessing the Risk of Groundwater Contamination From Household Wastewater Treatment

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Assessing the Risk of Groundwater Contamination From Household Wastewater Treatment

Farm•A•Syst: Farmstead Assessment System Worksheet #6
Included when you order this worksheet: MU publication WQ680, Reducing the Risk of Groundwater Contamination by Improving Household Wastewater Treatment, the fact sheet that corresponds with this worksheet.

Virtually all farms use a septic system or similar on-site wastewater treatment system. Although these systems generally are economical and safe, household wastewater can contain contaminants that degrade water quality for such uses as drinking, stock watering, food preparation and cleaning.

Potential contaminants in household wastewater include disease-causing bacteria, infectious viruses, household chemicals and excess nutrients, such as nitrate. Viruses can infect the liver, causing hepatitis. They also can infect the lining of the intestine, causing gastroenteritis (vomiting and diarrhea). If coliform organisms (a group of indicator bacteria) are found in your well water, they show that the water is potentially dangerous for drinking and food preparation. Septic systems are potential sources of bacterial contamination.

The quantity of wastewater also can present an environmental concern. Too much water entering the home treatment system reduces the efficiency of the system and can shorten its life.

Your drinking water is least likely to be contaminated if you follow appropriate management procedures or dispose of wastewater in any location that is off the farm site. However, proper off-site disposal practices are essential to avoid risking contamination that could affect the water supplies and health of others.

The goal of Farm•A•Syst is to help you protect the groundwater that supplies your drinking water

How will this worksheet help me protect my drinking water?

It will take you step by step through your household wastewater treatment practices.

  • It will rank your activities according to how they might affect the groundwater that provides your drinking-water supplies.
  • It will provide you with easy-to-understand rankings that will help you analyze the "risk level" of your household wastewater treatment practices.
  • It will help you determine which of your practices are reasonably safe and effective and which practices might require modification to better protect your drinking water.

How do I complete the worksheet?

Follow the directions at the top of the following chart. It should take you about 15 to 30 minutes to complete this worksheet and figure out your ranking.
 

Table 1
Household wastewater treatment: Assessing drinking-water contamination risk

  1. Use a pencil. You may want to make changes.
  2. For each category listed on the left that is appropriate to your farmstead, read across to the right and circle the statement that best describes conditions on your farmstead. (Skip and leave blank any categories that don't apply to your farmstead.)
  3. Then look above the description you circled to find your "rank number" (4, 3, 2 or 1) and enter that number in the blank under "your rank."
  4. Directions on overall scoring appear at the end of the worksheet.
  5. Allow about 15 minutes to 30 minutes to complete the worksheet and figure out your risk ranking for well-management practices.
  Low risk, rank 4 Low to moderate risk, rank 3 Moderate to high risk, rank 2 High risk, rank 1 Your rank
Quantity of wastewater Conservative water use (less than 20 gallons per person per day). Good maintenance of water-conserving fixtures. Whole house use is less than design capacity.1 Moderate water use (20 to 60 gallons per person per day). Fair maintenance of fixtures. Some water-conservation fixtures. Water softener recharges twice a week or less. Whole house use is near design capacity.1 High water use (60 to 120 gallons per person per day). Poor maintenance of fixtures. Water softener recharges more than twice a week. Whole house use occasionally exceeds design capacity.1 Excessive water use (greater than 120 gallons per person per day). Leaking fixtures. No water-conserving fixtures. Whole house use frequently exceeds design capacity.1  
Quality of wastewater
Settleable solids No use of garbage disposal unit in kitchen sink. Minimal use of garbage disposal unit (1 to 2 times per week). Moderate use of garbage disposal unit (3 to 5 times per week). Daily use of garbage disposal unit.  
Dissolved solids Minimal use of household chemicals (cups per week). No disposal of solvents and toxic cleaning agents. No water softener, or not recharged on site. Careful use of household chemicals (pints per week). Minimal disposal of solvents and toxic cleaning agents. Water softener used, recharged on site. Moderate use of household chemicals (quarts per week). Moderate disposal of solvents and toxic cleaning agents. Extensive use of household chemicals (gallons per week). Extensive disposal of solvents and toxic cleaning agents.  
Floatable solids No disposal of grease or oils into sewer. Domestic wastes only. Minimal disposal of grease or oils. Oil and grease wiped from cooking utensils before washing. Moderate disposal of grease or oils. No attempt to reduce disposal of grease and oil from household, but little generated. Extensive disposal of grease or oils.  
Collection of wastewater All wastewater collected for treatment. No clear water collected. No leakage loss of water that should be treated. No settling of soil near tank or collection system. Collection system (pipe) more than 50 feet from well. All wastewater wastewater collected for treatment. Some clear water collected. No leakage loss of water that should be treated. Some wastewater diverted, or some leakage of water that should be treated, and clean water infiltration. Clear water infiltration. Leakage loss of water that should be treated. Collection system (pipe) less than 50 feet from well.1,2  
Pretreatment system
Lagoon       Any lagoon or direct discharge of water.2  
Or         Or
Septic tank   Serial tanks or added retention system. No leakage. Pumped at least every 3 years and maintained. Baffles checked. Tanks checked; no leakage. Single tank. Pumped at 4- to 6-year intervals. Leakage losses. Seldom pumped out (greater than 7-year intervals). Less than 100 feet from well1,2 Less than 3 feet from saturation or bedrock.  
Or         Or
Packaged aerobic system Maintenance program followed. Loaded at less than design capacity.3 No mechanical failures. Loaded near design capacity. Occasional failures Frequent system failure. Load exceeds design capacity.3  
Or         Or
Holding tank Excess capacity for usual pumping interval. 300 feet down-slope from well. Tanks checked; no leakage. Excess capacity for pumping interval. 100 to 300 feet upslope from well. Tanks checked; no leakage.2 Occasional overflow or leakage. Less than 100 feet downslope.2 Less than 50 feet from well. 1Leakage losses. Upslope from well.2  
Additional treatment (all systems) Aeration, denitrification, filtration and disinfection. Aeration and/or denitrification. Filtration and/or disinfection. No additional treatment.  
Disposal of wastewater
Subsurface application (septic system or other treatment systems) Off-site disposal Pressure or gravity-fed distribution to trench system. Bed or seepage pit. Field or silo tile drainagesystem. Pipe to surface.2  
Or         Or
Surface application (holding-tank wastes) Off-site disposal Sufficient storage to accommodate best application time. Incorporated. Approved disposal site. Frequent application. No incorporation. Approved disposal site. Pit, agricultural field or surface drainage system; or outlet pipe or holes in holding tank.2  
Horizontal separation of wastewater disposal site from water supply (subsurface or surface) Off-site disposal Subsurface disposal down-slope more than 100 feet from well. Surface disposal more than 300 feet from well. Subsurface disposal downslope less than 50 feet from well. Surface disposal less than 200 feet from well.2 Upslope from well  
Vertical separation of wastewater disposal site from water supply (subsurface) Off-site disposal More than 6 feet to saturated soil or bedrock. More than 3 feet to saturated soil or bedrock. Less than 3 feet to saturated soil or bedrock.2  
Subsurface application rate (septic system or other treatment systems) Off-site disposal.   Below design capacity. At or above design capacity.  
Or         Or
Surface application rate (holding-tank wastes) Off-site disposal. Less than 170 gallons per acre per week. Vegetation harvested. Nitrogen application doesn't exceed plant uptake and harvesting. Less than 170 gallons per acre per week. Vegetation harvested. Nitrogen application exceeds plant uptake and harvesting. More than 170 gallons per acre per week
No vegetation harvesting.
 
Soils Off-site disposal. Medium- or fine-textured soils (silt loam, loam, clay loams, clays). Medium- to course-textured soils (sandy loam, sands). Very coarse sands or gravel.  
If you have only a holding tank for wastewater disposal, skip to the bottom of the chart and total your score
Disposal of pumpage from septic tanks and other treatment systems, except holding tanks
Surface application Off-site disposal. Sufficient storage to accommodate best application time. Incorporated. Approved disposal site. Frequent spreading. No incorporation. Approved disposal site. Non-approved site
 
 
Horizontal separation from water supply Off-site disposal. Downslope more than 300 feet. Downslope 100 to 300 feet
 
Upslope or less than 100 feet from water supply.  
Vertical separation from water supply Off-site disposal. More than 15 feet to saturated soil. 4 to 15 feet to saturated soil or bedrock Less than 4 feet to saturated soil or bedrock.2  
Soils Off-site disposal. Medium- or fine-textured soils (silt loam, loam, clay loams, clays). Medium- to course-textured soils (sandy loam, sands). Very coarse sands or gravel.  
Surface application rate Off-site disposal. Less than 85 gallons per acre per week. Vegetation harvested. Nitrogen application does not exceed plant uptake and harvesting. Less than 250 gallons per acre per week. No vegetation harvested. Nitrogen application exceeds plant uptake and harvesting. More than 250 gallons per acre per week
No vegetation harvested.
 
Use this total to calculate risk ranking in Equation 1. Total:  
1Illegal for new well installation. Existing wells must meet separation requirements in effect at time of construction.
2Besides representing a higher-risk choice, this practice also violates Missouri law.
3Low-permeability soils, such as clay, allow water to flow through slowly. High-permeability soils, such as sand and gravel, allow much faster water movement.

What do I do with these rankings?

Step 1
Begin by determining your overall well management risk ranking using Equation 1. Total the rankings for the categories you completed, and divide by the number of categories you ranked:

Equation 1

________ divided by ________ equals ________
(total of rankings)   (number of categories ranked)   (risk ranking1)
1Carry your answer out to one decimal place.
If your risk ranking is Your risk is
3.6 to 4 low
2.6 to 3.5 low to moderate
1.6 to 2.5 moderate to high
1 to 1.5 high

This ranking gives you an idea of how your household wastewater practices as a whole might be affecting your drinking water. This ranking should serve only as a general guide, not a precise diagnosis. Because it represents an average of many individual rankings, it can mask any individual rankings (such as 1s or 2s) that should be of concern. (Step 2.)

Enter your household wastewater risk ranking above in the first table in Worksheet #9 (MU publication WQ659). Later you will compare this risk ranking with other farmstead management rankings. Worksheet #8 (MU publication WQ658) will help you identify your farmstead's site conditions (soil type, soil depth and bedrock characteristics) and Worksheet #9 (MU publication WQ659) will show you how these site conditions affect your risk rankings.

Step 2
Look over your rankings for individual activities:

  • Low-risk practices (4s)
    ideal; should be your goal despite cost and effort
  • Low- to moderate-risk practices (3s)
    provide reasonable groundwater protection
  • Moderate- to high-risk practices (2s)
    inadequate protection in many circumstances
  • High-risk practices (1s)
    inadequate; pose a high risk of polluting groundwater

Regardless of your overall risk ranking, any individual rankings of "1" require immediate attention. Some concerns you can take care of right away; others could be major — or costly — projects, requiring planning and prioritizing before you take action.

Step 3
Read Fact Sheet #6 (MU publication WQ680), Improving Household Wastewater Treatment, and consider how you might modify your farmstead practices to better protect your drinking water.

Household wastewater treatment glossary

These terms may help you make more accurate assessments when completing Worksheet #6. They also may help clarify some of the terms used in Fact Sheet #6 (MU publication WQ680).

  • Approved disposal site
    A site for land application of wastewater or tank pumpage that meets state standards and is approved by the Department of Natural Resources.
  • Clear water infiltration
    Entry of water into a system that does not need treatment, such as rainfall or tile drainage, through unsealed joints, access ports and cracks.
  • Design capacity
    Maximum volume of liquid that can be treated in a particular wastewater treatment system. For systems that include subsurface wastewater disposal and distribution, capacity also is based on the soil's ability to accept and treat sewage effluent. In filling out the worksheet, if you don't know the design capacity of your system, use 150 gallons per bedroom per day as an estimate.
  • Effluent
    Liquid discharged from a septic tank or other treatment tank.
  • Holding tank
    An approved watertight receptacle for the collection and holding of sewage.
  • Hydraulic loading rate
    The volume of waste discharged per unit per area per time.
  • Off-site disposal
    Disposal of wastewater of sludge off the farm, such as at a municipal treatment plant or approved disposal site.
  • Scum
    Floatable solids, such as grease and fat.
  • Seepage pit (dry well)
    Underground receptacle constructed to permit disposal of septic-tank effluent, treated wastes or clear wastes by soil absorption through its bottom and walls.
  • Sludge
    Settleable, partially decomposed solids resulting from biological, chemical or physical wastewater treatment.
The Missouri Farmstead Assessment System is a cooperative project of MU Extension; College of Agriculture, Food and Natural Resources; and the Natural Resources Conservation Service.
The National Farmstead Assessment Program provided support for development of the Missouri program. These materials are adapted from the Wisconsin and Minnesota prototype versions of Farm•A•Syst.
This material is based upon work supported by the Extension Service, U.S. Department of Agriculture, under special project number 91-EHUA-1-0055 and 91-EWQI-1-9271.
Adapted for Missouri from material prepared by Susan Jones, U.S. E.P.A., Region V, Water Division, and University of Wisconsin Cooperative Extension.
MU Extension Farm•A•Syst team members: Joe Lear, Regional Agricultural Engineering Specialist and Chief Editor; Beverly Maltsberger, Regional Community Development Specialist; Robert Kelly and Charles Shay, Regional Agricultural Engineering Specialists; Thomas Yonke, Program Director, Agriculture and Natural Resources; Jerry Carpenter, State Water Quality Specialist; and Bob Broz, Water Quality Associate.
Technical review provided by August Timpe, Missouri Department of Natural Resources; Charles Fulhage, MU Department of Agricultural Engineering; U.S. E.P.A. Region VII, Environmental Sciences Division; and Missouri Natural Resources Conservation Service.
Information derived from Farm•A•Syst worksheets is intended only to provide general information and recommendations to farmers regarding their own farmstead practices. It is not the intent of this educational program to keep records of individual results.

WQ656, new October 1995


WQ656 Assessing the Risk of Groundwater Contamination From Household Wastewater Treatment | University of Missouri Extension