7 Del. Admin. Code § 7103-70.0
Current through Register Vol. 28, No. 3, September 1, 2024
Section 7103-70.0 - Nitrogen Balance/Cover Crop Selection and Management70.1 General 70.1.1 Nitrate concentration in percolate from wastewater irrigation systems must not exceed the state drinking water standard of 10 mg/L. Percolate nitrate concentration is a function of nitrogen loading, cover crop, management of vegetation and hydraulic loading. The design wastewater loading determined from water balance calculations must be checked against nitrogen loading limitations. If for the selected cover crop and management scheme, the proposed wastewater loading results in estimated percolate nitrate concentrations exceeding 10 mg/L, either the loading must be reduced or a cover crop with a higher nitrogen uptake must be selected.70.2 Nitrogen Balance 70.2.1 Percolate nitrate concentrations are estimated from a monthly nitrogen balance based on the average design wastewater loading, proposed cover crop, and cover crop management scheme.Example nitrogen balance calculations are presented in Section 7.0.70.2.2 In nitrogen balance calculations, all nitrogen not lost to denitrification, ammonia volatilization or plant uptake is assumed to leach into the groundwater as nitrate. For row and forage crop systems, assumed losses to denitrification should not exceed 15 percent of the total nitrogen applied. In forest systems, assumed denitrification losses should not exceed 25 percent. (Forest denitrification losses may be greater because more carbon is generally available in forests and the hydraulic loading rate can often be greater.(c) Assumed losses to ammonia volatization should not exceed 5 percent of the total ammonia applied. Some industrial wastes with high pH may experience higher rates of ammonia volatilization. Soil storage of nitrogen should be assumed to be zero. The Department recommends Tables 4-11 and 4-12 of the United States Environmental Protection Agency's October 1981 Process Design Manual: Land Treatment of Municipal Wastewater" for guidance in selecting cover crops and their nutrient uptake rates. For forests, the most recent estimates of nitrogen uptake are presented in Cole, Henry, and Nutter (1986), Forest Alternative for Land Treatment of Municipal and Industrial Wastes. In all cases, the source of the plant nitrogen uptake rate used for design must be referenced in the Design Development Report.70.3 Cover Crop Selection and Management70.3.1 Row crops may be irrigated with wastewater containing domestic waste only when not intended for direct human consumption. Forage crops irrigated with domestic wastewater must be harvested before feeding to livestock. Unmanaged, volunteer vegetation (i.e. weeds) is not an acceptable spray field cover. Disturbed areas in forest systems must be initially grassed and replanted or covered with a thick mulch for succession to forest vegetation.70.3.2 Spray field cover crops require management and periodic harvesting to maintain optimum growth conditions assumed in design. Forage crops must be harvested and removed at least several times annually. Pine forest systems should be harvested at 20 to 25 year intervals. Hardwood forest systems should be harvested at 40 to 60 years. It is recommended that whole tree harvesting be considered to maximize nutrient removal. However, due to soil exposure and compaction as a result of the harvesting activities, wastewater loadings must be reduced until the hydraulic capacity of the site is restored. Spray field area to allow for harvesting and the regeneration cycle should be considered in design.70.3.3 Many wastewaters are usually deficient in some of the essential fertilizer elements needed for vigorous agronomic cover crop growth. High growth rate forage crops may require supplemental fertilizer addition to maintain nutrient uptake rates assumed in design. Industrial wastewaters considered for irrigation should be carefully evaluated for their plant nutrient values.7 Del. Admin. Code § 7103-70.0