Water Isotopes in the South Willamette Valley Ground Water Management Area: a tool for determining water sources

Transcription

1 Water Isotopes in the South Willamette Valley Ground Water Management Area: a tool for determining water sources J. Renée Brooks Western Ecology Division, U.S. Environmental Protection Agency

2 What can we learn from using water isotopes in SWV-GWMA? What are the water sources containing nitrate? Do nitrate levels vary because of water source shifting or some other reason? Do the water isotopes indicate information about residence time?

3 Brief Lesson on Stable Isotopes of Water Stable Not radioactive Do not change with time Natural tracers found in all water (H 2 O) Hydrogen isotopes (d 2 H) Oxygen isotopes (d 18 O)

4 Isotopes of an element vary in the number of neutrons No charge change: Behaves the same chemically. Greater Mass: Moves Slower (diffusion) Chemical Bonds are Stronger.

5 Water isotopes are partitioned by hydrologic fluxes Water isotopes change with phase changes: Vapor Liquid (precipitation and evaporation) Do not change as water flows

6 d 2 H Isotopes in Precipitation Strong rain-out effect across basin Orographic lift Elevational gradient in water isotopes Brooks et al Ecosphere 3:39

7 Mean Monthly Flow (m 3 s -1 ) Flow dynamics from the three sources Willamette River Brooks et al Ecosphere 3:39 <200 m m > 1200 m 0 Oct 08 Feb 09 Jun 09 Oct 09 Feb 10 Jun 10 Oct 10

8 What can we learn from using water isotopes in SWV-GWMA? What are the water sources containing nitrate? Do nitrate levels vary because of water source shifting or some other reason? Do the water isotopes indicate information about residence time?

9 GWMA Well Monitoring 19 Drinking water (DW) 26 Ground water (GW) Measured Quarterly Nitrate, DO, Temp, Cond, S, ph Water isotopes added in May sampling dates

10 River Valley GW Nitrate (mg L -1 ) Drinking Ground d 2 H ( )

11 Seasonal Nitrate Variation 20 GW-22 GW-11 GW-20 GW-5 15 Nitrate (mg L -1 ) /1/06 2/1/07 8/1/07 2/1/08 8/1/08 2/1/09 8/1/09 2/1/10 8/1/10 2/1/11 8/1/11

12 Nitrate varies with shifting source water 10 River May 2014 Valley GW 8 May 2012 Nitrate (N mg L -1 ) 6 4 May 2013 Feb Aug 2012 Feb 2014 Aug 2013 May Aug Nov 2013 May 2012 Nov Aug 2012 Nov Aug 2012 May 2014 May Nov Nov 2012 May Feb Feb Aug Nov Feb 2013 DW-7 GW-5 GW H ( )

13 Two Neighboring Wells: one shifting source, one not DW-1525 DW May 2012 Nitrate (N mg l -1 ) Aug 2012 Feb 2013 May 2013 May Nov 2012 Aug Nov Feb Aug May Aug 2012 Feb 2013 May 2014 Nov May 2014 Nov 2012 Valley GW Unknown H ( )

14 River Influenced Wells 8 wells are influenced by the Willamette River. 5 wells switching sources 2 wells River water Seasonal d 2 H variance River pattern but lower amplitude Short residence time River Water = Low Nitrate Well Values Located near Willamette River

15 Varying Nitrate, 35 but not Source Water River Valley GW Nitrate (N mg L -1 ) GW-3 DW-11 GW-22 GW-10 GW-12 GW-11 GW H ( )

16 Varying Nitrate, Stable Water Isotopes do not indicate more than one water source Different sources with the same isotopic signature? Well depth varies a lot in these wells. Nitrate Variation Denitrification process? Dilution? Nitrate Isotopes (d 15 N and d 18 O) might help. Located further from the Willamette River.

17 10 River Stable Wells Valley GW Nitrate (N mg L -1 ) DW-10 GW-13 GW-21 DW-12a DW-3 GW-7 GW-6 DW-9 GW-9 GW-24 GW-8 DW-5 DW H ( )

18 Stable Wells Isotopes stability indicate one water source Nitrate stability indicates no processing Long residence times for well water source Located further from the Willamette River

19 Evaporated Water d 2 H ( ) d 18 O ( )

20 Evaporated Water Water isotopes vary with evaporation of source water. Lake Water? Irrigation Water? Wetlands? Found in multiple locations in the Valley. Evaporated source may or may not have nitrate. Nitrate concentration mostly does not relate to the degree of evaporation Variance overtime indicates groundwater is not well mixed. Residence time are likely short.

21 Stable Water Isotopes in Groundwater Well Monitoring Separated wells with valley bottom water and Willamette River water Nitrate was found in wells with valley bottom water Explained the variation in nitrate for some wells, not others Source water switching Indicates residence time of the well water Variation in isotopes indicates short residence time Constant values indicate long residence time

22 Implications for Management Wells with Short Residence Time Current management practices likely have an impact. Might be able to see improvements overtime. Evaporated Water Identify possible local source for the groundwater (Lake, wetland, or irrigation) If the well has high nitrate, identify possible sources around groundwater source.