Assessing Decadal Trends of a Nitrate-Contaminated Shallow Aquifer in Western Nebraska Using Groundwater Isotopes, Age-Dating, and Monitoring
Abstract
:1. Introduction
2. Materials and Methods
2.1. Site Description
2.2. Sample Sites
2.3. 3H/3He Sampling and Noble Gas Modeling
2.4. Recharge Estimates
2.5. Nitrate Isotopes, Nitrate, Ammonium, and DOC Concentrations
2.6. Evaluating Long-Term Trends in Nitrate Concentrations
3. Results and Discussion
3.1. Groundwater Age-Dating
3.2. Recharge Rates
3.3. Nitrate Analysis
- Analysis of a long-term dataset from much broader groundwater collection efforts in the Dutch Flats area, including sporadic sampling between 1998 and 2016, and intensive sampling in 1998, 1999, 2008, and 2016 (Table 3, Figure 5 and Figure 6, and Appendix A (Table A1)).
3.4. Sources of Nitrate
3.5. Biogeochemical Processes
3.6. Analysis of Other Relevant Environmental Variables
3.7. Further Discussion
4. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Appendix A
Well ID | 1990s [NO3−] * (mg N L−1) | 2008 [NO3−] (mg N L−1) | 2016 [NO3−] (mg N L−1) |
---|---|---|---|
10A-S | 1.4 | 0.0 | 0.0 |
10E-S | 16.5 | 5.4 | 0.0 |
10K-S | 7.6 | 3.9 | 4.4 |
10M-S | 9.5 | 4.5 | 6.4 |
10N-S | 3.0 | 2.1 | 0.0 |
1C-S | 9.9 | 8.7 | 9.5 |
1E-S | 13.7 | 13.4 | 33.5 |
1G-S | 8.6 | 9.7 | 34.5 |
1H-S | 11.3 | 8.1 | 13.7 |
1J-S | 16.3 | 11.4 | 13.8 |
1M-S | 10.9 | 10.4 | 16.1 |
1N-S | 3.3 | 2.1 | 4.3 |
2E-S | 4.9 | 3.2 | 4.7 |
2F-S | 16.1 | 0.9 | 30.5 |
2J-S | 12.4 | 8.0 | 8.8 |
3B-S | 2.6 | 0.3 | 0.0 |
3C-S | 1.8 | 0.0 | 3.8 |
3F-S | 4.8 | 27.7 | 0.5 |
4A-S | 0.5 | 0.5 | 0.7 |
5A-S | 2.9 | 2.3 | 1.4 |
5B-S | 0.9 | 1.9 | 2.2 |
5D-S | 11.1 | 8.8 | 6.5 |
5F-S | 4.7 | 4.6 | 3.5 |
5G-S | 11.4 | 13.3 | 19.8 |
6C-S | 4.8 | 5.6 | 5.9 |
6D-S | 19.2 | 20.9 | 26.4 |
6E-S | 8.9 | 5.1 | 6.3 |
6F-S | 3.6 | 0.7 | 0.9 |
6G-S | 2.1 | 1.8 | 1.7 |
6H-S | 3.6 | 4.3 | 3.5 |
6M-S | 4.5 | 8.1 | 6.4 |
6N-S | 6.9 | 9.1 | 5.7 |
7A-S | 5.3 | 3.8 | 3.3 |
7B-S | 6.5 | 8.7 | 2.6 |
7C-S | 14.3 | 14.2 | 11.2 |
7D-S | 3.3 | 1.3 | 5.0 |
7H-S | 14.2 | 10.6 | 17.8 |
8B-S | 5.3 | 4.6 | 11.3 |
8C-S | 5.6 | 4.7 | 5.7 |
8D-S | 5.4 | 4.7 | 6.5 |
8E-S | 0.1 | 0.0 | 0.0 |
8G-S | 1.4 | 1.8 | 5.0 |
9D-S | 5.4 | 1.2 | 0.0 |
9E-S | 9.6 | 8.7 | 6.4 |
11D-M | 2.7 | 2.3 | 2.2 |
1C-M | 3.0 | 4.8 | 5.7 |
1E-M | 9.0 | 7.1 | 4.6 |
1G-M | 9.6 | 4.0 | 7.1 |
1H-M | 4.0 | 3.1 | 3.5 |
1J-M | 7.8 | 9.2 | 4.0 |
1M-M | 11.9 | 5.9 | 12.9 |
2C-M | 2.3 | 1.6 | 2.3 |
2D-M | 5.0 | 1.9 | 1.3 |
2F-M | 19.8 | 12.1 | 21.4 |
2J-M | 9.4 | 8.9 | 4.7 |
2L-M | 4.8 | 4.8 | 1.4 |
3C-M | 1.1 | 1.3 | 0.9 |
3E-M | 7.9 | 10.0 | 9.2 |
3F-M | 6.2 | 12.1 | 0.0 |
5B-M | 0.9 | 0.6 | 0.6 |
10A-D | 3.6 | 6.5 | 9.4 |
10E-D | 5.2 | 5.1 | 4.1 |
10K-D | 5.0 | 5.3 | 6.2 |
10M-D | 9.4 | 5.9 | 7.6 |
11A-D | 4.8 | 3.9 | 4.2 |
1C-D | 2.4 | 3.0 | 4.3 |
1E-D | 2.3 | 3.6 | 3.1 |
1G-D | 8.9 | 4.0 | 6.2 |
1L-D | 4.9 | 6.3 | 1.1 |
2C-D | 2.8 | 2.4 | 1.9 |
2D-D | 1.4 | 1.2 | 1.4 |
2F-D | 3.1 | 3.1 | 3.8 |
2L-D | 1.6 | 1.3 | 1.0 |
3B-D | 1.2 | 0.5 | 0.3 |
3C-D | 1.2 | 1.1 | 1.6 |
3E-D | 3.7 | 3.7 | 3.9 |
3F-D | 11.5 | 5.7 | 4.7 |
5B-D | 0.9 | 0.8 | 0.6 |
6G-D | 6.2 | 1.3 | 2.0 |
6H-D | 0.1 | 0.9 | 0.6 |
6M-D | 2.4 | 1.1 | 1.4 |
7A-D | 1.9 | 1.7 | 1.7 |
7C-D | 5.8 | 9.4 | 5.7 |
7D-D | 3.7 | 3.3 | 4.1 |
8D-D | 5.7 | 5.4 | 8.1 |
9D-D | 2.8 | 3.1 | 3.1 |
9E-D | 5.5 | 4.6 | 3.4 |
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Well ID | Böhlke et al. [11] | Current Study | ||||||
---|---|---|---|---|---|---|---|---|
Depth (m) * | GW Age (years) | Recharge—Linear (m/year) | Recharge—Exp. (m/year) | Depth (m)* | GW Age (years) | Recharge—Linear (m/year) | Recharge—Exp. (m/year) | |
1E-S | 2.4 | 5.4 | 0.16 | n.d. | 1.9 | 4.9 | 0.13 | n.d. |
2D-S | 4.9 | 2.8 | 0.61 | n.d. | 2.4 | 5.4 | 0.15 | n.d. |
1E-I | 15.7 | 20.2 | 0.27 | 0.38 | 14.5 | 20.9 | 0.24 | 0.34 |
2D-I | 25.8 | 11.3 | 0.80 | 1.2 | 22.6 | 20.5 | 0.39 | 0.56 |
1C-D | 20.3 | 12.0 | 0.59 | n.d. | 19.4 | 12.0 | 0.57 | n.d. |
1E-D | 29.4 | 31.5 | 0.33 | n.d. | 28.3 | 47.0 | 0.21 | n.d. |
1L-D | 28.5 | 20.5 | 0.49 | n.d. | 30.3 | 20.2 | 0.53 | n.d. |
2D-D | 44.3 | 20.9 | 0.74 | n.d. | 41.1 | 23.9 | 0.60 | n.d. |
Mean: | 15.6 | 0.50 | 19.3 | 0.35 | ||||
Std. Dev.: | 9.5 | 0.23 | 13.3 | 0.19 |
Well ID | Böhlke et al. [11] | Current Study | |||||
---|---|---|---|---|---|---|---|
Date Sampled | δ15N-NO3− (‰) | [NO3−] (mg N L−1) | Date Sampled | δ15N-NO3− (‰) | [NO3−] (mg N L−1) | δ18O-NO3 (‰) | |
1G-S | 27 August 1998 | 2.4 | 8.8 | 18 April 2016 | 17.0 | 46.8 | n.d. |
1G-I | 27 August 1998 | n.d. | 10.6 | 18 April 2016 | 2.6 | 7.2 | n.d. |
1G-D | 27 August 1998 | 2.5 | 8.0 | 18 April 2016 | 3.3 | 6.2 | n.d. |
2D-S | 27 August 1998 | 5.7 | 8.3 | 16 August 2016 | 0.5 | 9.7 | −4.5 |
2D-I | 27 August 1998 | 5.6 | 5.1 | 16 August 2016 | 2.2 | 1.3 | −9.16 |
2D-D | 27 August 1998 | 4.9 | 1.4 | 16 August 2016 | −2.9 | 1.4 | −6.96 |
1E-S | 24 August 1998 | 2.9 | 15.8 | 16 August 2016 | −1.6 | 45.2 | −5.45 |
1E-I | 24 August 1998 | 2.7 | 10.8 | 16 August 2016 | −1.3 | 3.6 | −6.6 |
1E-D | 24 August 1998 | 4.1 | 2.5 | 16 August 2016 | −3.7 | 3.1 | −5.37 |
1L-D | 25 August 1998 | 10.2 | 2.4 | 17 August 2016 | 1.1 | 1.1 | −3.67 |
1C-I | 27 August 1998 | 4.2 | 2.5 | 17 August 2016 | 4.9 | 5.3 | −7.27 |
1C-D | 24 August 1998 | 4.5 | 2.5 | 17 August 2016 | −2.0 | 3.8 | −8.38 |
1G-S | 27 August 1998 | 2.4 | 8.8 | 12 October 2016 | 18.4 | 22.1 | 4.08 |
1G-I | 27 August 1998 | n.d. | 10.6 | 12 October 2016 | 9.5 | 6.9 | 0.33 |
Well Depth | Shallow (n = 44) | Intermediate (n = 16) | Deep (n = 27) | ||||||
---|---|---|---|---|---|---|---|---|---|
1990s * | 2008 | 2016 | 1990s * | 2008 | 2016 | 1990s * | 2008 | 2016 | |
Mean | 7.2 | 6.2 | 8.0 | 6.6 | 5.6 | 5.1 | 4.0 | 3.5 | 3.5 |
Median | 5.4 | 4.7 | 5.3 | 5.6 | 4.8 | 3.7 | 3.6 | 3.3 | 3.4 |
Mann–Whitney Test | Years | p-value | Years | p-value | Years | p-value | |||
1990s *–2008 | 0.15 | 1990s *–2008 | 0.66 | 1990s *–2008 | 0.68 | ||||
1990s *–2016 | 0.49 | 1990s *–2016 | 0.17 | 1990s *–2016 | 0.62 | ||||
2008–2016 | 0.70 | 2008–2016 | 0.38 | 2008–2016 | 0.94 |
Variable | Mean (±std) | p-Value | |
---|---|---|---|
Year: 1983–1999 | Year: 2000–2016 | ||
Precipitation (mm) | 431 (±97) | 370 (±118) | 0.11 |
Interstate Canal Discharge (km3/year) | 0.52 (±0.08) | 0.44 (±0.08) | 0.007 * |
Planted Corn Area (hectares) | 29,471 (±2568) | 34,217 (±2608) | <0.001 * |
Year: 1987–1999 | Year: 2000–2012 | ||
Fertilizer Loads (kg) | 11,503,061 (±1,150,187) | 9,540,057 (±1,222,507) | <0.001 * |
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Wells, M.J.; Gilmore, T.E.; Mittelstet, A.R.; Snow, D.; Sibray, S.S. Assessing Decadal Trends of a Nitrate-Contaminated Shallow Aquifer in Western Nebraska Using Groundwater Isotopes, Age-Dating, and Monitoring. Water 2018, 10, 1047. https://doi.org/10.3390/w10081047
Wells MJ, Gilmore TE, Mittelstet AR, Snow D, Sibray SS. Assessing Decadal Trends of a Nitrate-Contaminated Shallow Aquifer in Western Nebraska Using Groundwater Isotopes, Age-Dating, and Monitoring. Water. 2018; 10(8):1047. https://doi.org/10.3390/w10081047
Chicago/Turabian StyleWells, Martin J., Troy E. Gilmore, Aaron R. Mittelstet, Daniel Snow, and Steven S. Sibray. 2018. "Assessing Decadal Trends of a Nitrate-Contaminated Shallow Aquifer in Western Nebraska Using Groundwater Isotopes, Age-Dating, and Monitoring" Water 10, no. 8: 1047. https://doi.org/10.3390/w10081047