ADVANCEMENT OF THE SCIENCE
soils. Both Long (1995) and Roeder (2008) used soil types to estimate total nitrogen reductions from OWTS discharges. The hydrologic soil group (HSG), as defined by the Natural Resources Conserva- tion Service (NRCS, 2019), is divided into four groups (A, B, C, and D) and is used in MEANSS to account for soil-related anoxic conditions and the relative amount of carbon in soils. Although NRCS uses additional cri- teria for the HSG designation, the amount of clay is an important part of the designation and generally uses the following criteria: • Group A soils have <10% clay materials. • Group B soils have 10–20% clay materials. • Group C soils have 20–40% clay materials. • Group D soils have >40% clay materials. Soils with higher clay content tend to have more carbon and thus can provide an envi- ronment that is better for denitrification. Clay soils also have lower permeability rates and lower porosity, which slows the wastewa- ter migration rate and allows more time for denitrification to occur. As such, MEANSS uses the HSG to estimate the relative amount of clay and carbon in the soil and correlates increased denitrification rates to higher soil clay content. Travel time in the environment (primar- ily in groundwater) is another factor that has been correlated to denitrification. Because nitrate persists in the environment, it has more time to encounter conditions conducive to denitrification (Kroeger et al., 2006). In MEANSS, however, distance is used instead of travel time because it is easier to measure dis- tances than the three parameters that control groundwater travel time: hydraulic gradient, hydraulic conductivity, and eective porosity. Increased distances from surface water also allow for deeper groundwater flow, which increases the chances of encountering anoxic conditions that are conducive to denitrifica- tion (Dubrovsky et al., 2010). Review of the existing literature presented in this article provided three factors that are used in MEANSS to estimate denitrification: 1) the predominant HSG beneath the drain- field, 2) the predominant HSG in the riparian zone of the receiving surface water, and 3) the distance between the drainfield and the receiving surface water. The nitrogen reduc- tion values applied to these characteristics are presented in the Parameters subsection within the Methods section.
TABLE 1
Nitrate Attenuation Factors for Onsite Wastewater Treatment System Discharges to Soil
% Nitrate Reduction *
Scoring Category 1
Scoring Category 2
Scoring Category 3
Hydrologic Soil Group at Drainfield
Hydrologic Soil Group Within 30.5 m of Surface Water
Distance to Surface Water (m)
0
A B C D
A
0–30.5
10 20 30 50
30.6–152.5 152.6–1,525 1,526–6,100
B C D
≥6,101
*The total nitrate reduction is the sum of the individual reductions for Category 1 + Category 2 + Category 3. For example, a drainfield in a hydrologic group C soil (20% reduction) that drains to a surface water with hydrologic soil group B riparian soil (20% reduction) and is 100 m from the surface water (10% reduction) would reduce its nitrate load to the surface water by 50% from the original load discharged from the drainfield (Figure 1).
in each soil type. The relation between soil CaCO 3 and phosphorus adsorption is valid for soils with typical pH values. In some soils that have unusually high pH values (typi- cally >8), however, phosphorus adsorption is higher in calcareous soils than similar soils with more neutral pH values (Buckman & Brady, 1972). When these high-pH soil con- ditions exist, the MEANSS user can adjust and increase the amount of attenuation in the analysis for the calcareous soils. Using a similar logic as described for nitrate, distance is used as a criterion for phosphorus attenuation. For the same dis- tance, however, a larger amount of reduc- tion is applied to phosphorus than nitrate in MEANSS. Phosphorus is treated dier- ently because wastewater plumes with high phosphorus concentrations are less mobile than nitrogen, have been found to extend a relatively short distance from the source, and create high concentrations of phosphorus in soils immediately below drainfields with low levels beyond that location (Gold & Sims, 2000; Lombardo, 2006; Makepeace & Mlad- enich, 1996; Reneau et al., 1989; Robertson et al., 1998). Riparian areas, where anaerobic conditions often exist that are conducive to denitrifica- tion, provide a poor environment for soil adsorption and precipitation of phosphorus (Vought et al.,1994). Therefore, riparian soil
Phosphorus Attenuation Factors Phosphorus has lower mobility than nitro- gen and is removed in soils by two primary processes: adsorption and precipitation. The vadose zone is considered the primary loca- tion for phosphorus attenuation due partially to the negative soil moisture potentials that push the treated wastewater into the finer soil interstices and promote phosphorus adsorp- tion and precipitation (U.S. Environmental Protection Agency [U.S. EPA], 2002). Finer- grained soils also tend to impede phosphorus migration more than coarser soils primarily due to their greater surface area that provides more locations for adsorption. The HSG of the predominant soil beneath the drainfield is used to determine the relative amount of fine-grained soil. MEANSS does not distin- guish between precipitation and adsorption; instead, it applies a single reduction factor combining the two processes. Noncalcareous soils impede the movement of phosphorus more than calcareous soils because calcareous soils commonly main- tain neutral pH levels where phosphorus precipitation does not readily occur (Lom- bardo, 2006; Lusk et al., 2011; Robertson et al., 1998). Lombardo (2006) defined calcare- ous soils as soils containing >15% CaCO 3 and noncalcareous soils as those containing <1% CaCO 3 . MEANSS uses these CaCO 3 divisions to adjust the amount of attenuation occurring
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Volume 86 • Number 8
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