Nitrate Load Reduction Strategies for the Raccoon and Des Moines Rivers. Keith Schilling, Calvin Wolter Iowa DNR Geological and Water Survey

Transcription

1 Nitrate Load Reduction Strategies for the Raccoon and Des Moines Rivers Keith Schilling, Calvin Wolter Iowa DNR Geological and Water Survey

2 Outline of Presentation Background of nitrate impairments Nitrate reductions required by TMDL SWAT modeling scheme Nitrate reduction strategies evaluated for: i) Raccoon River and ii) Des Moines River Concluding remarks Acknowledgements: Phil Gassman and Manoj Jha, CARD, Iowa State; Allen Bonini, Watershed Improvement Section, Iowa DNR

3 Raccoon and DSM rivers as a drinking water source Des Moines Water Works is a public water supply serving Des Moines metropolitan area of 4, people DMWW source water includes surface water collected directly from the Raccoon and Des Moines Rivers Water sources are designated as Class C as a raw water source of potable water supply so drinking water standards are applicable The applicable water quality standard for nitrate for Class C designated use is the USEPA maximum contaminant level (MCL) of 1 mg/l. Drinking water or Gulf of Mexico hypoxia.still nitrate

4 Impaired segments

5 Watershed Characteristics

6 Nitrate impairment Raccoon River Flow Max. 4Load Maximum % of Days Mean Max. Mean Mean Point Range Flow 1 in Exceedance Reduction Needing Reduction Nitrate mg/l NPS Source Range Factor 1 Needed Reduction Needed Load Contrib. Contrib. (cfs) 3 (%) 2 (%) 2 (Mg) 1mg/l (%) 3 (%) Measured 97.8 N load Concentrations 1.8 > 1 mg/l 1.4 = 24% Daily N load (Mg/day) Discharge (cfs) Nitrate Concentration (mg/l) 1- (all data) Multiplication factor to assess degree of nitrate load exceedance (i.e., existing load in 1-9 range exceeds TMDL by factor of 1.93). 2 Reductions determined for only those days with an exceedance. 3 5 Nitrate source contributions determined for only those days with an exceedance Flow percentile

7 Nitrate impairment Des Moines River Flow Range (%) Daily N load (Mg/day) 1 Minimum Flow 15 in Range (cfs) 1 Discharge (cfs) % 77.6% 98.8% 1.2% % 57.8% 97.9% 2.1% % 27.6% 96.2% 3.8% % 18.5% 94.% 6.% % 5.7% 91.1% 8.9% Concentrations 115 > 1 mg/l 29.2% = 16.4% 4.6% 86.3% 13.7% %.9% 83.9% 16.1% All data % 19.3% 97.1% 2.9% 2 Nitrate Concentration (mg/l) Maximum Flow in Range (cfs) Maximum Mean NPS Mean Point Reduction Measured % of Days N load Load Source Load Needed Target Needing load with Contribution MOS (9.5 mg/l) Contribution (%) Reduction (%) (%) Flow percentile Calander Year

8 SWAT Modeling System Input Description Source of Data Physically based and continuous watershed scale hydrology 3-m and digital water elevation quality model 22 Developed 15-m landcover to predict grid impacts of land management practices on watershed hydrology and water quality 12-digit Hydrologic Unit Code boundaries HRUs represent percentages of subwatersheds with common soil, land use and management characteristics Daily Subbasins climate established at HUC12 level phtml Soil survey Animal feeding operations Census data

9 112 subbasins 364 HRUs 173 subbasins 2516 HRUs

10 Tile Manure Drainage Sources

11 Model Calibration - Flow Annual Streamflow (in) Raccoon River Measured Simulated Annual Discharge (mm) Des Moines River Measured Simulated Monthly Streamflow (in) Measured Simulated Monthly Discharge (mm) Measured Simulated Year Month and Year Calibration statistics r 2 =.84, E =.83 (month) r 2 =.8, E =.79

12 Model Calibration Nitrate Annual NO 3 -N Load (kg/ha) Monthly NO3-N Load (kg/ha) Measured Simulated Monthly Nitrate Load (Mg) Nitrate Load (Mg) Year Measured Simulated Measured Simulated Month and Year Calibration statistics r 2 =.53, E =.48 (month) r 2 =.77, E =.74

13 Nonpoint Nitrate Loading Patterns Des Moines River nitrate loads: 8 subbasins > 2 kg/ha Max = 28.6 kg/ha Average = 17.5 kg/ha Raccoon River nitrate loads: 11 HUC12 subbasins >3 kg/ha Average = 25.1 kg/ha

14 Nitrate Load Reduction Strategies for the Raccoon River Reduce the rate of ammonia fertilizer application in the watershed to 15 kg/ha, 1 kg/ha and 5 kg/ha Remove all cattle from the streams. Remove all human waste from the watershed. Land use change. 1. Convert all row crop lands located on slopes greater than B slopes to CRP grassland. 2. Convert all row crop lands located on floodplain alluvial soils to CRP.

15 Reduction in Nitrate Loads from Baseline Condition (%) Nitrate Load Reductions from baseline condition Annual Annual Percent 1 Nitrate Nitrate 16 Change Scenario Load Load from (Mg) (tons) Baseline Baseline 2 condition 17,43 19,173 14% Reduce fertilizer from 17 to 15 kg/ha (152 to ,436 18,8-5.7% lbs/ac) 3 12 Reduce fertilizer from 17 to 1 kg/ha (152 to 89 14,118 15,53-19.% lbs/ac) Reduction (%) = -.24x Reduce 4fertilizer from 17 to 5 kg/ha (152 to 45 12,218 13, % lbs/ac) r 2 = No cattle 5 in streams 17,325 19,58 -.6% No human waste 15,722 17, % kg/ha Annual Nitrate Load (Mg) lbs/ac Fertilizer Application Rate

16 Targeted land use change Two options: 1. Convert crop ground on HEL to CRP 2. Convert crop ground on floodplains to CRP Two scales: 1. Entire Raccoon River basin 2. South Raccoon River basin Scenario Annual Nitrate Load (Mg) Annual Nitrate Load (tons) Percent Change from Baseline Baseline condition 17,43 19,173 % Convert crop ground on C slopes or greater to CRP 15,878 17, % Convert crop ground on alluvial soils to CRP 16,837 18, % Land use change % N reduction Land area Ratio HEL to CRP Floodplain to CRP

17 South Raccoon Land Use Change Baseline Convert Row Crop on HEL to CRP Convert Row Crop on Alluvial Soils to CRP Watershed area (km 2 ) Row Crop ground (km 2 ) % of watershed in Row Crop 63.% 4.3% 55.5% Row Crop ground converted (km 2 ) % of Row Crop converted 36.1% 11.9% % of watershed converted 22.7% 7.5% NO3 load (kg) 2,954, 2,13, 2,489, NO3 loss (kg/watershed ha) NO3 loss (kg/rc ha) NO3 load reduction (kg) 851, 465 % NO3 load reduction 28.8% 15.7% Converting alluvial soils from row crop to CRP results in nearly 1.7 times more nitrate load reduction than converting row crop on HEL to CRP

18 Beyond the TMDL, more Raccoon River scenarios Additional scenarios evaluated by Jha, Wolter, Schilling and Gassman, TMDL analysis with SWAT modeling for the Raccoon River watershed, Iowa, 28 SWAT model calibrated and scenarios evaluated at Van Meter gage Global land use and management changes assessed 16 scenarios evaluated within five general categories, including converting CRP to continuous corn (ethanol scenario)

19 Scenario 1 (convert grasslands to continuous corn) 2 (convert croplands to grasslands) Description Average annual nitrate load (Metric Tons) Percent change from baseline - Baseline condition 23,51 - CRP to continuous corn 24, all grasslands (CRP, hay and pasture) to continuous corn 26, percent of row crops to CRP 18, percent of row crops to CRP 13, percent of row crops to CRP 8, all of row crops to CRP 3, (decrease nitrogen fertilizer N fertilizer application rate 1 kg/ha 15, application) N fertilizer application rate 5 kg/ha 12, N fertilizer application rate 15 kg/ha 2, (remove point sources) cattle from the streams 23, human waste (septic and WWTPs) 21, pastured cattle (no grazing) 23, (remove livestock) Cattle from feedlots 22, CAFOs 18, all livestock 17,

20 Nitrate Load Reduction Strategies for the Des Moines River Three global-scale nitrate load reduction scenarios were evaluated: 1. Reduce the rate of ammonia fertilizer application in the watershed to 1 kg/ha and 5 kg/ha (89 and 45 lbs/ac, respectively). 2. Remove all manure generated from permitted or registered CAFOs and feedlots. 3. Remove all human waste from the watershed.

21 Global-scale changes Baseline Condition (17 kg/ha, manure, humans) 1 kg/ha Ammonia Fertilizer 5 kg/ha Ammonia Fertilizer No manure No human waste Total kg NO 3 -N Load 28,95, 21,66, 17,95, 26,85, 27,55, Reduction (kg NO 3 -N) 7,29, 11,, 2,1, 1,4, Percent Reduction (%) 25.2% 38.% 7.3% 4.8% Ammonia in Fert kg/ha Ammonia N reduction Percent Reduction (%) 37.96% 65.8% 7.81% NO 3 in Fert (kg/ha) Organic N in Fert (kg/ha) Total N in Fert (kg/ha) Reduced N in Fert (%) 29.5% 5.6% 12.1% Reduction Ratio 85.3% 75.1% 59.8%

22 Nitrate Load Reduction Strategies for the Des Moines River Four strategies for targeting load reductions in the basin: 1. Target major nitrate load reductions in all subbasins with annual average losses greater than 15 kg/ha (55 subbasins out of 173). 2. Target major nitrate load reductions in all subbasins of the Boone River watershed. 3. Target major nitrate load reductions in subbasins located closest to the DMWW intake at 2nd Avenue. 4. Target major nitrate load reductions in subbasins located furthest away from the DMWW intake at 2nd Avenue (Minnesota subbasins).

23 Minnesota basins 55 basins > 15 kg/ha Boone River basin Lower DSM basins

24 Nitrate Reductions from Targeting Baseline (17 kg/ha, manure, humans) Top 55 subbasins reduced to 5kg/ha Boone River watershed reduced to 5 kg/ha Minnesota subbasins reduced to 5 kg/ha Lower DSM subbasins reduced to 5 kg/ha Total NO 3 -N Load (kg) 28,95, 24,88, 27,37, 27,2, 27,39, Reduction (kg NO 3 -N) 4,7, 1,58, 1,75, 1,56, Percent Reduction (%) 14.6% 5.46% 6.4% 5.39% Ammonia in Fert kg/ha Ammonia N reduction Percent Reduction (%) 2.76% 9.54% 9.81% 7.29% NO 3 in Fert (kg/ha) Organic N in Fert (kg/ha) Total N in Fert (kg/ha) Reduction in Fert. N (%) 16.14% 7.42% 7.63% 5.67% Reduction ratio based on N applications 87.12% 73.56% 79.27% 95.1% Area of watershed affected (ac) 4,3,35 1,22,258 58,27 611, ,811 Percentage of DSM River watershed 3.28% 14.4% 15.18% 13.74% Reduction ratio based on land area 46.43% 37.92% 39.79% 39.23%

25 Concluding remarks 1. TMDL calls for nitrate reduction of 48% in Raccoon River and 34% in Des Moines River can this be achieved? 2. SWAT model useful for evaluating global load reduction scenarios and variations on targeting 3. In both Raccoon and DSM basins, nitrate is primarily a nonpoint source issue (point sources <1% with generous assumptions) 4. Raccoon River basin, modeling suggests major land use change or major fertilizer reductions needed to achieve reductions 5. Are floodplains targets of conservation opportunity? 6. Future land cover change to more corn will increase nitrate loads and make TMDL compliance more difficult 7. In the DSM River basin, model results suggest global scale reductions in fertilizer application (everyone pitching in) achieved greater nitrate load reduction than targeting. (Reducing to 1 kg/ha by everyone was better than 55 subbasins reducing to 5 kg/ha) 8. If targeting is preferred, reducing fertilizer applications was most efficient near the watershed outlet