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Home My WebLink About20160191 Ver _Information Letter_20090422 GREENVILLE SOUTHWEST BYPASS INDIRECT AND CUMULATIVE IMPACT WATER QUALITY STUDY UPDATE Comments from the NCDWQ staff on the report submitted by NCDOT regarding the Indirect and Cumulative Impacts (ICI) associated with the Greenville Southwest Bypass (Greenville Bypass) in Pitt County, North Carolina (TIP R-2250) included concerns about the assumptions regarding septic tank failure rates in the original watershed modeling analysis. Concerns were also raised in regards to the geographic extent of stream buffers projected in the watershed model to meet requirements of the Neuse River NSW strategy. Over and above the concerns raised by NCDWQ, changes in land use patterns projected to result from development of the Greenville Bypass have occurred since the ICI study was completed as a result of the elimination of one of the roadway interchanges in the bypass design (Figure 1). In order to address NCDWQ comments and account for the impacts of these land use changes, additional model runs were performed. This memo, which serves as an addendum to the original Greenville Southwest Bypass ICI Water Quality Report (NCDOT 2006b), contains the model results as well as a discussion regarding the impacts of adding streams shown on the Pitt County Soil Survey. NEW MODELING SCENARIOS The Generalized Watershed Loading Function (GWLF) model was used to simulate long-term loading of nonpoint source pollutants. Two additional runs of the model were completed in order to address septic tank failure rates and the elimination of an interchange. Full details on the previous model assumptions and parameters can be found in the original report (NCDOT, 2006b). Septic T8171Fai/111-0 Flates The GWLF model incorporates four different septic tank 'types' in order to account for septic tank failure. 'Normal' systems conform to EPA guidelines, in which nitrogen entering surface water is assumed to be a factor of plant uptake or its ability to infiltrate groundwater and subsequent discharge to streams. Phosphorus is assumed to be completely absorbed by soils in this scenario. In 'short-circuit'. systems, the septic tanks are assumed to be in close-proximity to streams, and therefore phosphorus absorption by soil is assumed to be negligible. 'Ponded' systems describe septic tanks with hydraulic failure, resulting in the surfacing of tank effluent which enters surface water via overland flow. 'Direct discharge' systems are illegal systems which discharge tank effluent directly to surface water. Septic tank failure rates are applied in the GWLF model by adjusting the percent of the population which uses each 'type' of septic system. In the model performed for the original ICI study, 100% of the septic-using population was assumed to be using the 'normal' septic system. Statistical data on septic system malfunction in North Carolina is very limited. Data obtained for this study from the Pitt County Management Information Systems Office on septic repair permits resulted in a calculated septic failure rate of less than one percent. This is likely a gross underestimation of failure rate given the limited time' period of recorded failures (4 years) and that some systems with failure are never reported. Instead, a septic tank failure rate was assumed based on a state-wide survey performed by the North Carolina Office of State Budget and Management in 1981. In this citizen survey, 11.4% reported septic system malfunction or failure in the preceding year (NCDEH, 2000). Greenville Southwest Bypass ICI Water Quality Study Update US 264 _ l ,? 1 - --' Greenville r 1= i l 5 ?3 " 1 ``y(l ? 1 1 .. j ! 1 j ? R r 1 m r- L ? r?? POCOSIN RD r ? nterville ?? l 1-' GREENE C N µ -?Q• . ?. 'r ? NC 10 '. ? ` ? ? ? Qyden - ? l O Swift Creek Study Area Streams _I Municipalities O Little Contentnea Creek Study Area '- Impaired Streams /^/ Minor Roads Selected Alternative Water Bodies Major Roads { Interchange ! County Boundary l?f Railroads _ -,River Basin Boundary Figure 1. Project Study Area ICI Water Quality Study Greenville Soutwest Bypass ' TIP No. R-2250, P,f County, NC North Carolina Department of Transportation Pitt Count _ 0 0.5 1 2 3 Miles , 2 Greenville Southwest Bypass ICI Water Quality Study Update Given that the failure rate reported was based on homeowner observation, the most accurate representation of this failure in the GWLF model is. the 'ponded' system scenario. It is assumed that no illegal 'direct discharge' systems are present in the watershed. Using inputs developed for the Build scenario of the original Cl Ireport, 88.6% of the population on septic systems was input as using 'normal' septic systems and 11.4% on 'ponded' systems. The GWLF model was used to generate 10 years of annual total nitrogen (TN) and total phosphorus (TP) loads in the watershed. These loads were aggregated into 10-year pollutant loads of TN and TP for each subwatershed. The results are presented by pollutant in Table 1. Table 1. Ten-year Total Loads (tonnes) for All Subwatersheds in the 'Normal' and 'Normal + Pondin ' Se tic Failure Scenarios Total Nitrogen Total Phosphorus Subwatersheds Normal Normal + Pondin % Change: Over:Normal Normal Normal + `Pondin % Change Over Normal UF2 147 147 0% 9.06 10.52 16% UF1 58 58 0% 11.72 11.81 1% UE1 133 133 0% 7.24 8.64 19% UD1 17 17 0% 0.75 0.91 21% UC1 73 73 0% 3.53 4.22 20% U81 30 30 0% 1.23 1.50 22% UA3 165 165 0% 8.65 10.39 20% UA2 59 59 0% 10.51 10.73 2% UA1 55 55 0% 6.85 7.16 5% SC8 63 63 0% 9.47 9.59 1% SC7 53 53 0% 6.55 6.82 4% SC6 59 59 0% 10.58 10.64 1% SC5 67 67 0% 11.32 11.48 1% SC4 151 151 0% 14.23 15.41 8% SC3 79 79 0% 14.38 14.39 0% SC2 58 58 0% 10.58 10.69 1% SC1 92 92 0% 17.42 17.42 0% PB4 122 122 0% 5.81 7.07 22% PB3 70 70 0% 3.63 4.35 20% PB2 43 43 0% 3.29 3.68 12% PB1 53 53 0% 9.77 9.83 1% Total 1646 1646 177 187 6% The septic tank failure scenario demonstrated a 6% increase in TP loading in the entire study area watershed, with loading increasing between 0 and 22% among the subwatersheds (Table 1 and Figure 2). No change in TN loading was observed between the two scenarios. This is not surprising given the dynamics of the model and nature of the nitrogen cycle. In the model performed for the original ICI study, 100% of the septic- using population was assumed to be using the 'normal' type of septic system. In this system, nitrogen from septic tanks loads to surface water via infiltration to groundwater and its subsequent discharge to streams. In a conventional septic tank and drainfield system organic nitrogen in household wastes is transformed into ammonia products in the anaerobic conditions of the septic tank, a process referred to as ammonification. When these products exit the septic tank and encounter the aerobic conditions in the drainfield, the ammonia products are Greenville Southwest Bypass ICI Water Quality Study Update NC-9031171erchawge Removal Since the ICI study was completed in May of 2007, the previously planned NC Highway 903 interchange has been eliminated. To account for this design change, the Build scenario was re-evaluated (Figure 3 Project Subwatersheds). County officials had expected commercial and light industrial growth in this area as a result of the interchange (Figure 4a Original Future Build Land Use/Land Cover Scenario). In addition, higher density residential housing was anticipated in the area around the interchange. These predicted land uses have been replaced with the Residential - Low Density category which is consistent with the No-Build scenario. A revised Future Build Land Use/Land Cover Scenario has been created to depict these changes (Figure 4b Revised Future Build Land Use/Land Cover Scenario). To model the above scenario, the land use changes in the three subwatersheds affected by the interchange elimination (UF2, UE1, SC4) were incorporated into the previous model run developed to include septic tank failure rate. The. GWLF model was then used to generate time series reflecting 10 years of annual TN, TP, and sediment loads, which were aggregated to 10-year pollutant loads. The results are presented in Table 2. A 1% decrease in TN loading was observed in subwatersheds UF2 and UE1, with a 3% decrease in SC4, resulting in a study area decrease in TN loads of less than one percent (Figure 5). TP loads were decreased by 4%, 4%, and 8% in subwatersheds UF2, UE1, and SC4 respectively (Figure 6). The total reduction in TP loading in the study area was approximately 1%. Sediment loads also decreased as a result of the change in land use in the three subwatersheds. Loads were decreased by 5%, 7%, and 11% in each of, the respective subwatersheds, with a total study area decrease in sediment loads of approximately 1% (Figure 7). 5 Greenville Southwest Bypass Cl Water Quality Study Update GREENE Future Build Conditions EfflWater (% imperviousness) Commercial/Heavy Industrial (72%) -.:._ Open Space 8 Forest eV Selected Alternative. Figure 4a. Original Future Build Land Use/Land Cover Scenario Office/Institutional/Light Industrial (53%) ® Residential Multifamily/Very High Density (42%) Interchange Roads ICI Water Quality Study Greenville Southwest Bypass _ Residential High Density (24%) Railroads TIP No. R-2250, Pitt County, NC ® Residential Medium High Density (17%) - Residential Medium Low Density (13%) - esidential Low Density (11%) '?. Streams Watershed Boundary j County Boundary ' North Carolina epartment of Transportation ., j Agriculture/Residential Very Low Density (5%) - Paved Road with Right of Way (61%) L__j Ayden ETJ Greenville Proposed ETJ 0 0.5 1 2 3 Miles Wetland Winterville Proposed ETJ 7 Greenville Southwest Bypass ICI Water Quality Study Update Table 2. Ten-year Total Loads (tonnes) for All Subwatersheds in the Revised Septic and Revised Septic with Land Use Change Scenarios Total N itrogen . " Total Ph os horus TotalS ediment = . ` y 3 U) Revised Septic Revised Septic + LU Change 0. ._ o o Revised Septic Revised . Septic + LU Change 0 ° L u o o : Revised Septic;" Revised Septic + _ - -LU Change. = ' 2 .m ; -U o,• - UF2 147 145 -1% 11 10 -4% 468 495 -5% UF1 58 58 0% 12 12 0% 748 748 0% UP 133 132 -1% 9 8 -4% 422 454 -7% UD1 17 17 0% 1 1 0% 170 170 0% UC1 73 73 0% 4 4 0% 362 362 0% UB1 30 30 0% 1 1 0% 201 201 0% UA3 165 165 0% 10 10 0% 483 483 0% ' UA2 59 59 0% 11 11 0% 802 802 0% UA1 55 55 0% 7 7 0% 557 557 0% SC8 63 63 0% 10 10 0% 620 620 0% SC7 53 53 0% 7 7 0% 327. 327 0% SC6 59 59 0% 11 11 0% 667 667 0% SC5 67 67 0% 11 11 0% 707 707 0% SC4 151 146 -3% 15 14 -8% 626 700 -11% SC3 79 79 0% 14 14 0% 1004 1004 0% SC2 58 58 0% 11 11 0% 852 852 0% SC1 92 92 0% 17 17 0% 1312 1312 0% P64 122 122 0% 7 7 0% 424 424 0% PB3 70 70 0% 4 4 0% 243 243 0% PB2 43 43 0% 4 4 0% 185 185 0% PB1 53 53 0% 10 10 0% 771 771 0% Total 1646 1639 -0.4% 187 185 -1% 11951 12084 -1% Greenville Southwest Bypass ICI Water Quality Study Update Figure 7. Mean Annual Total Sediment Loading Rates Build 60 Revised Septic P0 Revised Septic + [3 - 50 LU Change e -' 40 r T r 30 rn Y - 20 - " C 10 0 STREAMS AND STREAM BUFFERS Comments from NCDWQ included concern over the quantity of streams, and therefore the amount of stream buffer, depicted on the land use scenarios. There was concern that the addition of streams would enable more of a constituent (nitrogen, phosphorous, and sediment) to enter the stream. Blue-line streams found on USGS topographic quadrangles were depicted on an overlay of the land use scenarios. Additional streams found on the Pitt County Soil Survey were not included in this overlay in the original study. GWLF models runoff in a block by block manner. Each block represents a subwatershed. Nutrient loads from different land uses within each subwatershed are based on the volumes of flow and the associated flow pathways (overland and seepage), the amounts of soil eroded, and concentrations that express the amount of nutrient load per unit volume of water flow or sediment erosion from each land use. As noted, the only two flow pathways are overland flow and seepage. GWLF does not model streamflow as a flow pathway, and therefore.the amount of streams found within the subwatershed is irrelevant as it will not affect the model outputs. The quantity of streams depicted does have an impact on land use within the subwatersheds. Specifically, the fifty-foot buffers that were created for each USGS blue-line stream were classified as urban greenspace. Urban greenspace is modeled as a rural land use with low concentrations of dissolved nitrogen and phosphorous. Eliminating these buffers would lead to greater amounts of urban land uses which would increase the nutrient loads in the system. Since the streams/channels depicted on the soil survey were not taken into account when developing the land use scenarios, the amount of buffer, and therefore urban greenspace, is most likely an underestimation. To illustrate this, additional streams/channels that appear on the soil survey in seven subwatersheds were digitized. Stream length was calculated, as well as the area of urban greenspace that the buffer would have occupied (Table 3). Results show that adding the soil survey streams lead to an increase of between 40 and 80% of stream length, 11 Greenville Southwest Bypass ICI Water Quality Study Update REFERENCES North Carolina Department of Transportation (NCDOT). 2006a. Final Technical Memorandum. Greenville Southwest Bypass Indirect and Cumulative Impact Analysis. Prepared for NCDOT by Lochner. May 2006. North Carolina Department of Transportation (NCDOT). 2006b. Greenville Southwest Bypass Indirect and Cumulative Impact Water Quality Study Report. Prepared for NCDOT by Stantec. December 2006. Washington State Department of Health. 2005. Nitrogen Reducing Technologies for Onsite Wastewater Treatment Systems. Report to the Puget Sound Action Team. DOH Pub 334-083. 13