Iowa Geological Survey, Trowbridge Hall, University of Iowa, Iowa City, IA. Iowa Soybean Association, Ankeny, Iowa

Transcription

1 Nitrate-nitrogen reductions measured in newly reconstructed Iowa oxbows located on and off the Des Moines Lobe, Iowa: Different hydrology, similar benefits Keith E. Schilling 1 *, Keegan Kult 2, Karen Wilke 3, Bryce Haines 4, Chris Jones 4 1 Iowa Geological Survey, Trowbridge Hall, University of Iowa, Iowa City, IA 2 Iowa Soybean Association, Ankeny, Iowa 3 The Nature Conservancy, Des Moines, Iowa 4 IIHR Hydroscience & Engineering, University of Iowa

2 Two monitoring sites:

3 Frye Oxbow site

4 Construction History Figure 1 Frye oxbow before, looking upstream. Figure 1 Frye oxbow immediately after excavation, water line is now just a few feet below the tile.

5 Construction History Figure 1 Frye oxbow beginning to fill, Figure 1 Frye oxbow re-vegetating on , looking downstream rather than upstream.

6 Geophysics Geophysical results using electromagnetic terrain conductivity (EM) (top panel) and electrical resistivity (ER) (cross sections).

7 Hydrologic Conditions

8 Nitrate Concentrations and Loads

9 Estimating Nitrate Retention Summed N inputs: Daily N or Cl input loads (kg/day) = Σ(TN + TS + GW + P) Groundwater inputs from Darcy s Law Used N:Cl ratios: Cl is a conservative tracer and loss of N relative to Cl concentrations suggests removal of N from the aqueous system from processes such as assimilation or denitrification Daily nitrate retention efficiency (%R) was calculated using the equation: %R = (1-(N/Cl ox /N/CL in ) x 100

10 N Retention In 2014, average retention efficiency was 44%. In 2015, the retention efficiency was 41%. During an overlapping May to September period, the efficiency was 44% and 47% in 2014 and 2015, respectively.

11 Seasonal Patterns Greater retention efficiencies was measured in late summer and early fall Maximum monthly retention values were 92% and 78% in August and September 2015.

12 Morgan Creek Oxbow site

13 Morgan Creek oxbow in eastern Iowa

14 Oxbow excavation in August 2016

15 Nitrate concentrations pre- and post-flood Morgan Creek Oxbow W1 W2 W3 W4 W5 W6 W7 W8 Preflood Postflood Mean St.dev Mean St.dev NO 3 -N concentrations (in mg/l) measured in Morgan Creek, reconstructed oxbow and water table wells (W1 to W8) during pre-flood period (June 29, 2016 to April 13, 2017; n = 12) and post April 15 flood (May 3 to June 2, 2017; n = 4).

16 Flooding history in Morgan Creek Figure 4.7. Water surface elevation of Morgan Creek from the years of 2011 to 2017 provided from IFC stream gauge in Meters above Mean Sea Level (masl). The black line shows flood elevation of the stream.

17 Spring flood of April 16 th, 2017 V = 3,500 cubic meters C = 5.35 mg/l Load = kg

18 Water levels and oxbow volume Figure 5.2. Depth to water in well 5 responding to flood event of April 2017

19 Installed nitrate sensor to monitor reduction in oxbow Nitratax sensor Concentrations decreased from 5.3 to 0.7 over 21 days

20 N mass reduction Denitrification rate was modeled using first order kinetics CC tt = CC 0 ee ( kkkk) kk = kk 20 θθ TT day period Retained: 0.30 g N m -2 day -1 Or 74% Why the different slopes? Oxbow is an open system N contributions from birds, fish, upgradient groundwater

21 Effectiveness of oxbows to reduce flood-delivered N 30-hour event carried ~10, 165 kg of NO 3 - N past the oxbow Retention of ~15 kg of NO 3 -N was 0.15% of the total event load

22 Comparing oxbow sites: Parameter Frye oxbow Morgan Creek Location Des Moines Lobe SIDP Stream White Fox Creek Morgan Creek Watershed size 19.5 km km 2 Stream NO3-N mg/l 8-12 mg/l Excavation Size 2,070 m 3 2,500 m 3 Groundwater seepage rate 0.25 m 3 /day K = 0.2 m/day 5.4 m 3 /day K = 4 m/day Primary N delivery Tiles Floods Mean N conc in oxbow 4-6 mg/l 0.5 mg/l N load reduction 220 to 470 kg/year 15 kg/one flood N retention efficiency 44-47% 74% Cost $15,000 $28,000 Frye oxbow Morgan Creek oxbow

23 Oxbows as BMP N reduction INRS BMPs for tile drainage included bioreactors (43 ± 21%), drainage water management (33 ±32%), wetlands (52%) and saturated buffers (50 ± 13%) Best comparison is with bioreactors (similar cost of construction ~$10-15,000 each) Landowners interested in oxbows water quality since wildlife responses (fish and waterfowl) are measurable and potential sites are typically found on marginal land unsuited for production For nutrient reduction: Reconstructing floodplain oxbows more effective to intercept high N loads from tile drainage water than flood flows

24 Questions July 2017