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expected to occur. Also shown are how increases in EXAMPLES OF ENVIRONMENTAL these environmental characteristics effect the magni- FATE CHARACTERISTICS WHICH tude of the anticipated attenuation and,therefore,the INFLUENCE ATTENUATION FACTORS selection of environmental attenuation factors. As the following characteristics increase... EFFECT ON ATTENUATION FACTOR If ground water is threatened by waste constitu- FOR THE PROTECTION OF GROUND WATER— INCREASE DECREASE ents,increases in the depth to ground water(thickness Depth to Highest Ground Water(incl.capillary fringe) • Net Recharge (i.e.,[rainfall]—[evaporation]) of the vadose zone),in the clay content,organic matter Characteristics of the Vadose Zone: a capacity or H of vadose zone Permeability and Porosity content,ion exchan g p •� p Clay Content materials,and in the ionic strep th,viscosity,de add- Organic Matter Content g � � Ion Exchange Capacity and pH bility or octanol/water partition coefficient(the affinity Pollutant Characteristics: _ Polarity of the chemical for octanol or soil organic matter versus Ionic Strength(more positive) Volatility(potential for vapor transport) its affinity for water)of the waste constituent will cause Viscosity Degradability or Biologic Activity the attenuation factor to be larger(greater attenuation Octanol/Water Partition Coefficient(Kow) 77,1777777 • Other Constituents that Could Increase Mobility expected). Increases in the net recharge rate(a driving Topography(steepness of terrain) • force for movement of waste constituents),in the per- Total Pollutant Load(mass loading)Uncertainty of the Data and Assumptions meability or porosity of vadose zone materials,in the polarity or volatility of the waste constituent,in the FOR THE PROTECTION OF SURFACE WATERS— concentrations of solvents or other chemicals that can • Distance from Drainage Courses - • Topography(steepness of terrain) increase the permeability of soils or act as carriers for • Pollutant Characteristics: Volatility(loss to atmosphere) the constituent,or in the mass loading of waste con- Reactivity or Degradability stituents will cause the attenuation factor to be smaller Polarity Octanol/Water Partition Coefficient(Kow) (less attenuation ex ected as the constituent mi rates Other Constituents That Could Increase Mobility P g Initial Dilution Upon Reaching Surface Waters to ground water). (min.surface water flow vs.max.pollutant flow) • Interconnection of Ground and Surface Waters If surface waters are threatened by constituents in a Total Pollutant Load(mass loading) • Uncertainty of the Data and Assumptions waste,increases in the distance of travel from the site of waste discharge to surface water,in the volatility, Figure 6 reactivity,degradability or octanol/water partition coefficient of the waste constituent,and in the amount tal attenuation should also be reflected in the amount of initial dilution that the waste or leachate would re- of vadose zone and ground water monitoring that is ceive upon entering surface waters will cause the at- required for a waste management unit. Greater uncer- tenuation factor to be larger. Increases in the steepness tainty necessitates a greater monitoring effort to assure of the terrain,in the polarity of the constituent,in the that the attenuation factor setting process was suffi- amount of interconnection between ground and sur- ciently protective of water quality. face waters,in the concentrations of solvents or other Site-and constituent-specific information regard- chemicals that can act as carriers for the constituent, ing key environmental fate characteristics under rea- and in the total constituent loading will lower the at- sonable worst-case conditions may be used to estimate tenuation factor. attenuation factors for specific waste constituents at a Undoubtedly the most important characteristic that site. Publications such as The California Site Mitigation must be evaluated in the derivation of environmental Decision Tree Manual from DISC,The Soil Chemistry of attenuation factors is the relative uncertainty of the Hazardous Materials by James Dragun,the USEPA pub- data and assumptions used to quantify environmental lications Superfund Exposure Assessment Manual,Water fate processes. The more uncertainty involved in the Related Environmental Fate of the 129 Priority Pollutants estimation of environmental attenuation factors,the and DRASTIC:A Standardized System for Evaluating more the assumptions being used in their derivation Ground Water Pollution Potential Using Hydrogeologic should lean toward underestimating the amount of Settings, and the Handbook of Environmental Data on attenuation expected to occur. In this way,a greater Organic Chemicals by Karel Verschueren contain infor-° assurance of water quality protection is provided. The mation and/or procedures that can be used to assess degree of uncertainty in the estimation of environmen- the fate of chemicals in the environment and to esti- Central Valley Regional Water Quality Control Board Page 5