David L. Rudolph University of Waterloo Department of Earth and Environmental Sciences

Transcription

1 Tools for Groundwater Supply and Quality Management in Irrigated Agriculture Groundwater Resources Association Of California First Annual Western Groundwater Congress September 25-27, 2018 Sacramento, California David L. Rudolph University of Waterloo Department of Earth and Environmental Sciences 1

2 Nature of ag-water quality management Quantifying nutrient management BMPs Influence of the vadose zone Monitoring challenges Case study: Utility of long term field monitoring Concept of decreasing denitrification potential Concluding comments 2

3 Complicated non-point contaminant source Nutrient application rates, seasonality, geology Influenced by legacy land use practices Hydrologic legacy (decadal response times) Biogeochemical legacy Monitoring strategies can be problematic 3

4 Nitrogen Cycle Root Zone Depth ~ 1m 1 Esmaeili (2013) 2 Vadose Zone Below Max. Root Depth Saturated Zone 3 4

5 Quantifying BMP Performance Are we making progress? 5

6 N City of Woodstock Primary water supply for City of Woodstock 5 production wells in sand and gravel aquifers. Thornton Well Field Average well depth 30 m 2 km Adjacent to active farm land where fertilizers applied for decades. 6

7 Nitrate (mg NO 3 -N/l) City of Woodstock Thornton Well Field Koch (2009) Public Well Nitrate Concentrations (Chronic) 7

8 Groundwater Management Strategy 1. Purchase agricultural land within the 2-year time of travel and implement nutrient management BMPs. 2. Quantify changes in nitrate leaching at the field scale (pre and post BMP). 3. Rely on BMPs to reduce nitrate concentrations in public supply wells. 8

9 Nutrient Management Strategy (2003) (111 ha acres) 38 ha 73 ha Thornton Well Field ~2-year time of travel (particle tracking in saturated zone) 500 m King and Wall (2006) 9

10 110 HA Surface nitrate mass flux 1) Recharge rate 2) Nitrate concentration Haslauer (2005); Bekeris (2007); Koch (2009); Esmaeili (2013), Rudolph et al. (2015) 10

11 N Core MET Station km Haslauer (2005); Bekeris (2007), Koch (2009) 11

12 Measurement Integration (1) tracer method (bromide) water and energy balance (MET) 1-D modeling tools (HELP3, SHAW, HYDRUS) (1) Scanlon, Healy, and Cook (2002). Choosing Appropriate Techniques for Quantifying Groundwater Recharge 12

13 Depth (m) Soil Bromide (mg/kg) Year Year Date 1 Date 2 8 Water Table 9 Depth (mbgs) Depth (m) Recharge = profile velocity x average water content Volumetric Moisture Content (%) Monthly Soil Moisture Profiles Bekeris (2007) Koch (2009) 13

14 Average Annual Precipitation: 1150mm/yr Koch (2009) 14

15 Replicate Vadose Zone Soil Coring Soil Sample Analysis NO 3, & Moisture Content Coring Locations Haslauer (2007); Bekeris (2007) 15

16 Post BMP Annual Nitrate Mass Flux > Recharge [mm/yr] Nitrate concentration [mg NO 3 -N/L] > Mass flux [g NO 3 -N/m 2 /yr] > < 1 Variability in recharge and nitrate mass loading over ~110 HA Koch (2009) 16

17 1. Avg. nitrate concentration beneath root zone decreased from ~ 20 mg/l to ~ 8 mg/l 2. Total nitrate mass loading decreased from 5.6 to 2.1 tonnes/year (from 2004 to 2009) * 60% reduction * Historic corn yields: 135 bu/ac Current corn yields: ~140 bu/ac! 17

18 Groundwater Quality in Monitoring Well Network Aquifer 2, May 2008 Aquifer 3, May 2008 Sep-Oct Oct-Nov May Minimum Average Maximum No. of wells Source Haslauer (2005) Koch (2009) Koch (2009) Sep-Oct Oct-Nov May Minimum Average Maximum No. of wells Source Haslauer (2005) Koch (2009) Koch (2009)

19 Groundwater Quality in Monitoring Well Network Aquifer 3, May 2008 Aquifer 3, August 2014 Sep 2004 Oct-Nov 2007 May 2008 Aug 2014 Minimum Average Maximum No. of wells Source Haslauer (2005) Koch (2009) Koch (2009) (unpublished)

20 Rudolph et al. (2015) 20

21 Implications and Conclusions 1. Nutrient reduction BMPs implemented on purchased land have been successful at reducing groundwater nitrate concentrations. Nitrate levels in Thornton wells have reduced significantly Crop yields have remained high 2. Water treatment infrastructure for nitrate removal was not required. 3. Full impact of the BMPs may take years to be realized. 21

22 Relative Reactivity f(x) NOM NOM NOM O 2 NO Release of electron donors from the matrix controls the overall reaction.

23 Decreasing Denitrification Potential NNOO 3 Natural Organic Matter (NOM) (Fe(II)-minerals) NNOO 3 Natural reaction potential decreases over time Accounting for the Decreasing Reaction Potential of Heterogeneous Aquifers in a Stochastic Framework of Aquifer-Scale Reactive Transport (WRR, 2018) Matthias Loschko, Thomas Wohling, David Rudolph, and Olaf Cirpka

24 Potential Implications Denitrification potential of natural aquifer sediments may progressively reduce as electron donors are used up Public wells currently protected by natural biodegradation of nitrate my become vulnerable to increasing concentrations over time 24

25 Collaborators Marcelo Sousa Loren Bekeris Claus Haslauer Jamie Koch Greg Padusenko Sara Esmaeili Andrew Wiebe Neil Thomson Brewster Conant Matthias Loschko Thomas Wohling Olaf Cirpka 25