Potential Nitrogen Loadings from On-site Wastewater Systems in Macro, Meso and Micro Scale

Transcription

1 Potential Nitrogen Loadings from On-site Wastewater Systems in Macro, Meso and Micro Scale Sushama Pradhan Michael T. Hoover Soil Science Dept., North Carolina State University, Raleigh, NC

2 Outline Introduction Objectives Macro scale - Materials and methods - Results and discussions - Conclusions Micro scale - Materials and methods - Results and discussions - Conclusions

3 Introduction A private wastewater treatment system for house 17 million (1970), 27 million (1990) Approximately 50 % of NC households use on-site wastewater treatment

4 Problems Lack of generally accepted model for assessing the contribution from OWS Existing sub basin nutrient models have typically ignored the nutrient inputs from OWS to NC river basins

5 Research objectives Macro and Meso scale watersheds Estimate cumulative potential nitrogen loadings from on-site systems to North Carolina s 17 river basins (Macro scale) and 134 sub-basins (Meso scale ).

6 Research objectives Micro scale watersheds Quantify fate and transport of nitrogen derived from on-site systems in small watersheds (Micro scale ).

7 Macro and meso scale methodology A GIS based area driven normalization procedure was developed and implemented to estimate potential nitrogen loadings from on-site systems. The total cumulative nitrogen was calculated here as the worst case scenario.

8 Data sets used Block group boundary layer for NC TIGER line files (US Dept. of commerce, Bureau of census 1990) Population and wastewater source census 1990 Watershed boundary for NC river basins NCCGIA ( North Carolina Center for Geographic and Information Analysis) Nitrogen loading published nitrogen estimates in septic system effluent (Nitrogen = 4.5 kg (10 lb) per person per year.

9 Census block group boundaries Generally contain 250 housing units to 550 housing units Never cross state or county boundaries May cross boundaries of county subdivision, urbanized area, voting district, congressional district and the boundary of any geographic entities

10 Watershed boundaries Defined by hydrological boundaries Not limited by political boundaries May cross census block, county, state and even international boundaries

11 Block group boundary and river basin boundary of North Carolina N River basin boundary Block group boundary Kilometers

12 Normalization of census block data by land area Assumption distribution of housing units using septic systems and the corresponding population was spatially uniform across the census block group Overlay of normalized census block group coverage and river basin coverage Potential nitrogen loading (watershed/subwatershed) = Population using septic system X 10 lb of nitrogen/yr

13 1. Normalization of census data by land area (100 septic systems) A B 10 septic systems/ ha 10 ha 10 ha A (50 septic systems) 5 septic systems/ ha 2. Overlay of census block and river basin coverage + = Census block group River basin Unionized

14 3. Total septic systems per intersected polygons septic systems 25 septic systems septic systems 4. Sum polygons on a unique identifier ID 1 ID 3 ID 2

15 Macro scale Results and discussion

16 Distribution of wastewater treatment systems Distribution of wastewater treatment systems Septic public Others Systems (%) 10 0 Hiwassee Little Tennessee Savannah Pasquotank Watauga New Broad Lumber Chowan Roanoke French Broad Yadkin Tar Pamlico White Oak Cape Fear Neuse Catawba NC wt.avg River basins/nc State

17 Density of septic systems for 17 River basins in North Carolina 1990 census block group data N Watauga New Roanoke Chowan Hiwassee Little Tennessee French Broad Broad Catawba Yadkin Cape Fear Neuse Tar-Pamlico Pasquotank Savannah White Oak Lumber Septic system density < 3.8/sq.km (< 10/sq.mi) /sq.km (10-40/sq.mi) > 15.4/sq.km (> 40/sq.mi)

18 Density of septic systems for 134 sub-basins in North Carolina 1990 census block group data N Watauga New Roanoke Chowan Hiwassee French Little Broad Tennessee Broad Catawba Yadkin Cape Fear Neuse Pasquotank Tar-Pamlico Savannah Lumber White Oak Sub-basin boundary River basin boundary Septic system density < 3.8/sq.km (< 10/sq.mi) /sq.km ( 10-40/sq.mi) > 15.4/sq.km (> 40/sq.mi) Kilometers

19 Cumulative potential nitrogen loading for 17 River basins in North Carolina 1990 Census Block Group Data N Watauga New Roanoke Chowan Hiwassee Little Tennessee French Broad Broad Catawba Yadkin Cape Fear Neuse Tar-Palmico Pasquotank Savannah White Oak Lumber County boundary Nitrogen loading (kg/yr) < 100, , , , ,999 1,000,000-1,499,999 1,500,000-2,000,000 > 2,000, Kilometers

20 N Cumulative potential nitrogen loading for 134 sub-basins in North Carolina 1990 census block group data Watauga New Roanoke Chowan Yadkin Tar-Pamlico Pasquotank French Little Broad Tennessee Broad Catawba Neuse Hiwassee Cape Fear Savannah Lumber White Oak River basin boundary Nitrogen loading (kg/yr) < 10,000 10,000-99, , , , ,000 > 300, Kilometers

21 Macro and meso scale conclusions 1. Total potential nitrogen loading = 39.6 million lb/year 2. Statewide usage of septic systems = 48% ; other system users = 2% 3. On a river basin scale housing units with septic systems ranged from 82% (Hiwassee) to 39% (Catawba, Neuse). 4. On a sub-basin level the percentage of septic system users ranged from 96% (WOK 5) to 10% (CT 34). 5. Distribution of septic systems (ranging from 5 systems/sq.km to 229 systems/sq. mi) on a sub-basin scale throughout the state.

22 Macro and meso scale vs. micro scale

23 N Hoods Creek watershed Kilometers 14 Sub-basin outlet Watershed outlet Houses Subbasins Soil Classes Autryville loamy sand Crotan muck Goldsboro loamy fine sand Leon sand Lynchburg fine sandy loam Masontown mucky fine sandy loam Norfolk loamy fine sand Pentego fine sandy loam Rains fine sandy loam State loamy sand Torhunta fine sandy loam &V Study site Hoods Creek watershed Lower Coastal Plan region Drains 179 ha 227 housing units 11 different soil classes 5 different LULC Field mass N-export data was available

24 Model selection 2003 version of SWAT (Soil and water assessment tool) model. Never has been used to predict fate and transport of nitrogen derived from on-site systems.

25 Model parameterization AVSWAT-X Soil Survey Geographic (SSURGO) data Digital Elevation Model (DEM) derived from Light Detection and Ranging (LIDAR) Digital Orthophoto

26 Soil dataset SSURGO dataset (1:24,000) STATSGO dataset (1:125,000) N N

27 Digital Elevation Model 5m x 5m LIDAR DEM 30m x 30m USGS DEM

28 Digital orthophoto Digitized houses

29 Nitrogen loading from OWS OWS was considered as a non-point source since SWAT model does not currently have a category for OWS N-input, effluent from OWS was applied as NO 3 -N fertilizer into the subsurface of the first layer of soil using management operation of model Nitrogen contributions/house/year = 2.5 people * 10 lb

30 Model simulation Multiple HRS/sub-basin 20% land use and 10% soil. Model was simulated from Jan 1 st 1994 to Dec. 31 st 2003.

31 Model simulation (cont.) Observed data from field investigations by Buetow (2002) and Humphrey (2002) in two sub-basins in the Hoods Creek watershed were used for comparison. Predicted and observed % of nitrogen lost for sub-basin 4 and 7 to the sub-basin outlet was compared.

32 Model calibration Flow - Curve number was adjusted within the range of tabulated values. Nutrient -Half life of nitrogen in ground water was modify by using field data.

33 Results and discussions

34 Fate and transport of nitrogen derived from OWS: sub-basin basin 4 N inputs (kg/ha/yr) N outputs (kg/ha/yr) % N lost HRU Septic system Rain F-MN A-MN DNIT Plant uptake NSUR NLATQ NGW Predicted Observed HRU 1: Area = 7.0 (ha), LULC = Urban medium low density, Soil type = Autryville loamy sand HRU 2: Area = 2.0 (ha), LULC = Urban medium low density, Soil type = Masontown mucky fine sandy loam URML = Residential medium low density, F-MN = Mineralization of fresh organic to mineral N, A-FM = Movement of nitrogen from the active organic pool to nitrate pool, DNIT = Denitrification, NUP = Plant uptake, NSURQ = Nitrate in surface runoff, NLATQ = Nitrate in lateral flow, NGE = Nitrate in base flow.

35 Fate and transport of nitrogen derived from OWS: sub-basin basin 7 N inputs (kg/ha/yr) N outputs (kg/ha/yr) % N lost HRU Septic system Rain F-MN A-MN DNIT Plant uptake NSUR NLATQ NGW Predicted Observed HRU 1: Area = 3.0 (ha), LULC = Urban medium low density, Soil type = Autryville loamy sand HRU 2: Area = 2.0 (ha), LULC = Urban medium low density, Soil type = Masontown mucky fine sandy loam URML = Residential medium low density, F-MN = Mineralization of fresh organic to mineral N, A-FM = Movement of nitrogen from the active organic pool to nitrate pool, DNIT = Denitrification, NUP = Plant uptake, NSURQ = Nitrate in surface runoff, NLATQ = Nitrate in lateral flow, NGE = Nitrate in base flow.

36 Nitrogen loading from OWS: watershed outlet N input from OWS (kg/yr) Nitrogen (kg/ha/yr) Lost thorough DNIT NUP Average nitrogen yield (kg/ha/yr) To sub-basin outlets from NSURQ NLATQ N0 3 GW Total N-load at watershed outlet (kg/yr) , DNIT= denitrification, NUP= plant uptake, NSURQ= surface run off, NLATQ= lateral flow, and N0 3 GW= base flow

37 Micro scale conclusions Over 95% of the nitrogen derived from OWS was removed within the Hoods Creek watershed prior to its discharge to the watershed outlet. 0.05kg of nitrogen/yr is contributed to watershed outlet by each system. Results may be different for other watersheds that have soils with less denitrification potential. Further validation is recommended.

38 Questions? New Roanoke Chowan Hiwassee Little Tennessee French Broad Broad Catawba Yadkin Cape Fear Neuse Pasquotank Tar-Pamlico Savannah Lumber White Oak