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L expected to occur. Also shown are how increases in these environmental characteristics effect the magni- tude of the anticipated attenuation and, therefore, the selection of environmental attenuation factors. If ground water is threatened by waste constitu- ents, increases in the depth to ground water (thickness of the vadose zone), in the clay content, organic matter content, ion exchange capacity or pH of vadose zone materials, and in the ionic strength, viscosity, degrada- bility or octanol/water partition coefficient (the affinity of the chemical for octanol or soil organic matter versus its affinity for water) of the waste constituent will cause the attenuation factor to be larger (greater attenuation expected). Increases in the net recharge rate (a driving force for movement of waste constituents), in the per- meability or porosity of vadose zone materials, in the polarity or Volatility of the waste constituent, in the concentrations of solvents or other chemicals that can increase the permeability of soils or act as carriers for the constituent, or in the mass loading of waste con- stituents will cause the attenuation factor to be smaller (less attenuation expected as the constituent migrates to ground water). If surface waters are threatened by constituents Ln a waste, increases in the distance of travel from the site of waste discharge to surface water, in the volatility, reactivity, degradability or octanol/water partition coefficient of the waste constituent, and in the amount of initial dilution that the waste or leachate would re- ceive upon entering surface waters will cause the at- tenuation factor to be larger. Increases in the steepness of the terrain, in the polarity of the constituent, in the amount of interconnection between ground and sur- face waters, in the concentrations of solvents or other chemicals that can act as carriers for the constituent, and in the total constituent loading will lower the at- tenuation factor. Undoubtedly the most important characteristic that must be evaluated in the derivation of environmental attenuation factors is the relative uncertainty of the data and assumptions used to quantify environmental fate processes. The more uncertainty involved in the estimation of environmental attenuation factors, the more the assumptions being used in their derivation should lean toward underestimating the amount of attenuation expected to occur. In this way, a greater assurance of water quality protection is provided. The degree of uncertainty in the estimation of environmen- Central Valley Regional Water Quality Control Board EXAMPLES OF ENVIRONMENTAL FATE CHARACTERISTICS WHICH INFLUENCE ATTENUATION FACTORS As the following characteristics increase... EFFECT ON ATTENUATION FACTOR FOR THE PROTECTION OF GROUND WATER— INCREASE DECREASE Depth to Highest Ground Water (incl, capillary fringe) Net Recharge (i.e., [rainfall] — [evaporation]) Characteristics of the Vadose Zone: Permeability and Porosity Clay Content Organic Matter Content Ion Exchange Capacity and pH Pollutant Characteristics: Polarity Ionic Strength (more positive) Volatility (potential for vapor transport) Viscosity Degradability or Biologic Activity Octanol/Water Partition Coefficient (Kow) Other Constituents that Could Increase Mobility Topography (steepness of terrain) Total Pollutant Load (mass loading) Uncertainty of the Data and Assumptions FOR THE PROTECTION OF SURFACE WATERS — • Distance from Drainage Courses • Topography (steepness of terrain) • Pollutant Characteristics: _ Volatility (loss to atmosphere) Reactivity or Degradability Polarity Octanol/Water Partition Coefficient (Kow) • Other Constituents That Could Increase Mobility • Initial Dilution Upon Reaching Surface Waters (min. surface water flow vs. max. pollutant flow) • Interconnection of Ground and Surface Waters • Total Pollutant Load (mass loading) • Uncertainty of the Data and Assumptions Figure 6 tal attenuation should also be reflected in the amount of vadose zone and ground water monitoring that is required for a waste management unit. Greater uncer- tainty necessitates a greater monitoring effort to assure that the attenuation factor setting process was suffi- ciently protective of water quality. Site- and constituent -specific information regard- ing key environmental fate characteristics under rea- sonable worst-case conditions may be used to estimate attenuation factors for specific waste constituents at a site. Publications such as The California Site Mitigation Decision Tree Manual from DTSC, The Soil Chemistry of Hazardous Materials by James Dragun, the USEPA pub- lications Superfund Exposure Assessment Manual, Water Related Environmental Fate of the 129 Priority Pollutants and DRASTIC: A Standardized System for Evaluating Ground Water Pollution Potential Using Hydrogeologie Settings, and the Handbook of Environmental Data on Organic Chemicals by Karel Verschueren contain infor- mation and/or procedures that can be used to assess the fate of chemicals in the environment and to esti- Page 5