7 Del. Admin. Code § 7103-23.0
Current through Reigster Vol. 28, No. 6, December 1, 2024
Section 7103-23.0 - Removal by DrainageAn important mechanism of removal of wastewater from a site is downward movement through the soil to the groundwater. To adequately estimate a hydraulic loading that will not result in site failure from overloading, one must consider the entire pathway from soil surface to outflow of groundwater through natural discharge areas or artificial drains. For analysis the pathway can be divided into three sections:
23.1 Infiltration and storage in the topsoil.23.2 Downward movement through the soil and parent material to the groundwater23.3 Movement of groundwater to an outlet and into surface water.23.4 Most Delaware soils consist of a fairly permeable A horizon over a B horizon with somewhat lower permeability. Irrigated water enters the A horizon at a relatively rapid rate and is temporarily stored there until it can move slowly through the B horizon or, on sloping areas, move laterally along the top of the B. During wet periods or with high loading rates, this laterally moving water may appear at seeps or springs at the bottom of a slope.23.5 In determining a hydraulic loading and an application rate, one must be certain that the soil has sufficient infiltration capacity and storage capacity in the A horizon to accept water, store it and then permit drainage at a rate which allows the soil to reaerate rather rapidly. Reaeration is necessary to assure aerobic decomposition of the organic material in the wastewater. Otherwise, anaerobic conditions will exist with subsequent nuisance odors and possibly sealing the soil by slimes formed during anaerobic decomposition.23.6 Infiltration capacity is a function of soil texture and structure, initial moisture content, vegetative cover, and temperature (e.g., frozen soil). Storage capacity is a function of pore size distribution.23.7 The rate of vertical movement through the soil is controlled by saturated hydraulic conductivity or permeability (Ksat) of the most restrictive soil layer. Procedures developed by EPA for estimating this downward movement use 4 to 10 percent of the Ksat of the restrictive horizon as the drainage rate (percolation) used in calculating allowable weekly or monthly hydraulic loading rate (7). EPA's procedures (7) for calculating design hydraulic loading rate uses a monthly water balance for precipitation, evapotranspiration (ET), and percolation (with adjustments for periods of non-operation due to management activities). The results of this monthly water balance should also be compared with the results of the monthly percolate nitrate concentration calculations to determine which is limiting, especially when vegetation is not present. Also, in some cases, the reaeration requirement and the A-horizon storage mentioned previously can be more limiting than the Ksat of the most restrictive layer. TABLE 2
Current USEPA Guidelines(5) For Zn, Cu, Ni, and Pb and Regulations(3) for Cd Application to Land Used for Production of Food-chain Crops
Soil Cation Exchange Capacity (meq/100g)(1)
| Metal | 0-5 | 5-15 | >15 |
| Cumulative Limit - lbc (Kg/ha) | | | |
| Pb | 500(560) | 1000(1120) | 2000 (2240) |
| Zn | 250(280) | 500 (560) | 1000 (1120) |
| Cu | 125(140) | 250 (280) | 500(560) |
| Ni | 125(140) | 250 (280) | 500(560) |
| Cd | 404(5) | 8.9 (10) | 17.8 (20) |
Annual Cd application rate not to exceed 0.44 lbc (0.5 Kg/ha)
NOTE: SOIL MUST BE MAINTAINED AT pH 6.5 OR ABOVE WHENEVER FOOD- CHAIN CROPS ARE GROWN UNLESS PLANT NUTRIENT NEEDS AND SOIL CHEMISTRY PRECLUDE SUCH VALUES WITHOUT EXCESSIVE LIME ADDITION, BASED NOT ON COST BUT ON UNREALISTIC TONNAGE OF LIME/ACRE. In such cases, lime additions suitable to the vegetation used are to be applied in conjunction with annual metal monitoring of the vegetation.
23.8 On sloping sites which have subsoils with lower values of Ksat, some of the applied water will move laterally above the zone of restricted Ksat. In this case water may move through a relatively small cross-sectional area. Thus, application rates have to be limited to prevent prolonged saturation of the topsoil down slope with subsequent surfacing of the water before adequate treatment has occurred.23.9 Rate of movement of groundwater from the site to an outlet is controlled by the groundwater gradient (i.e. difference in elevation of the groundwater under the site and at the outlet) and the Ksat of the material through which it is moving. The groundwater level under the site may rise due to irrigation leading perhaps to a mounding condition. One must select a loading such that the groundwater does not rise so close to the surface that wastewater does not receive adequate treatment prior to entering the groundwater. The EPA design manual on wastewater (7) gives methods for calculating: (a) subsurface drainage rates to surface water (ditches) and(b) groundwater mounding, or perched water table. Although these topics are presented in the EPA design manual mainly for high infiltration process design (very high application rates), the methods are applicable to slow rate systems as well.23.10 Drainage water moving to the groundwater also carries with it waste constituents not removed by storage in the soil or above-ground removal. To protect groundwater quality, concentrations of these constituents must not exceed allowable limits determined by groundwater classification (e.g. 10 mg/L for NO3--N in potable groundwater). Concentrations of waste constituents not specifically covered in these regulations must not exceed concentrations in the National Interim Primary and Secondary Drinking Water Regulations (2). The EPA design manual on wastewater irrigation (7) contains an annual mass balance method1 of determining loading rates which will prevent excess concentrations of NO3--N from entering the groundwater. This method can also be used for some other constituents as well, e.g. Cl- and SO4=.23.11 It is important that if a wastewater containing an imbalance of sodium to calcium and magnesium is applied that sodium not be allowed to accumulate appreciably on the cation exchange sites. If excessive accumulation of Na does occur, clay particles disperse and hydraulic conductivity is reduced. The dispersion hazard increases as the clay content of the soil increases and as the ratio of Na to (Ca + Mg) in the wastewater increases. This ratio, the sodium adsorption ratio (SAR), influences the extent of retention of Na on the exchange complex.23.12 Wastewater with an SAR greater than 15 will cause reduction in hydraulic conductivity in all but very sandy soils. Over long periods of time wastewaters with SAR's less than 15 can cause reduced hydraulic conductivity.23.13 1N draining to groundwater = N applied minus N removed by crop harvest, NH3 volatilization, and/or denitrification (i.e. above-groundremoval, Fig. 4-1).23.14 Gypsum (CaSO4= . 2H2O) may be used to supply a source of relatively soluble Ca to prevent Na accumulation in the soil. However, gypsum rates must be limited to those that will not raise groundwater SO4= concentrations above 250 mg/L. Municipal wastewaters with little industrial input rarely have SAR's above 2 to 5 and therefore do not represent a design limitation.7 Del. Admin. Code § 7103-23.0