7 Del. Admin. Code § 7103-22.0
Current through Register Vol. 28, No. 4, October 1, 2024
Section 7103-22.0 - Permanent Storage in Soil22.1 Most soils in Delaware have high P-fixing capacities because of their high Fe and/or Al content. Reaction products of P with Fe and Al are relatively insoluble so P does not move appreciably when applied to soil. The P fixation capacity of the soil is mainly a function of soil texture, to some extent drainage class, and previous P fertilization. The high P fixation capacity of Delaware soils will prevent P from leaching to the groundwater except in very sandy soils (e.g. Coastal Sussex) or at very high P loading rates. 22.2 A greater potential hazard than leaching to groundwater is P enrichment of surface water by P carried in eroded soil or runoff from fields receiving wastewater. Since the ratio of P/N in many wastewaters is much higher than the P/N ratio required by crops, application at rates to supply the N needs of the crop will result in application of large excesses of P. Consequently, the P concentration in topsoil will increase rapidly. 22.3 To minimize the potential P enrichment of surface waters, it is important that conservation measures be used to minimize erosion and adequate buffer zones be maintained between the site and surface water so sediment in runoff will be redeposited prior to reaching the watercourse. For this reason grass and forest vegetation are preferred over row crops that have exposed soil throughout the growing season. 22.4 Heavy metals react with a variety of soil constituents to form relatively insoluble compounds. The cation exchange capacity (CEC) is currently used as a measure of the soil's potential to tie up heavy metals. Table 2 presents current USEPA guidelines (and regulation for Cd) for land application of wastes containing heavy metals. Notice that soil pH must be kept at or above 6.5 for these rates to be used. 22.5 Since soils have finite capacities to retain phosphorus and trace metals, a useable site lifetime may be defined. For typical municipal wastewaters with low industrial input the site lifetime may exceed 100 years for phosphorus and several hundred years for trace metals. Site life for industrial wastewater systems must be evaluated on a constituent by constituent basis and site assimilative capacity for each. The concept of "site life" should not be interpreted to mean that at the end of the site life the land is no longer useable for agriculture. Quite the contrary. Sufficient safety factors have been incorporated into the guidelines so normal agricultural production can continue after waste additions are stopped.7 Del. Admin. Code § 7103-22.0