The U.S. Geological Survey (USGS) evaluated nitrate and orthophosphate concentrations in groundwater for temporal trends (monotonic and step trends) for the middle Snake River region (Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties) in south-central Idaho using the Regional Kendall test (monotonic trends) and the Wilcoxon signed rank test (step trends). The study evaluated two trend periods: 2000–09 and 2010–19/20. The study area was divided into six hydrogeologic zones (HZs) that had similar geologic and hydrologic characteristics and that correlated with county boundaries where possible. Two well networks sampled by the USGS National Water Quality Program within the HZs were also evaluated.
The northern Gooding County HZ had statistically significant increasing nitrate concentration trends for both the monotonic and step trends in the early trend period, while the Cassia and Jerome/Southern Gooding County HZs only had one of the statistical tests with statistically significant increasing nitrate concentrations. The Minidoka County HZ had conflicting results between the two statistical tests for the early time period with a statistically significant increasing monotonic trend in nitrate concentration and a statistically significant decreasing step trend. The differing results between these two statistical tests indicates the significance of concentration data during the middle of the time period. Both the Lincoln and Twin Falls County HZs did not have statistically significant trends for either test during either time period as well as the Northern Gooding County HZ for the latter time period. The Minidoka County HZ had statistically significant nitrate trends for both tests in the latter time period along with one of the trend tests for the Cassia and Jerome/Southern Gooding County HZ. Most of the nitrate concentration trend rates are low from 0.01 to 0.12 milligram per liter per year (mg/L/year) with the northern Gooding County HZ having the highest trend rate during the early time period of 0.28 mg/L/year for the step trend and 0.55 mg/L/year for the monotonic trend.
All the HZs and both well networks had statistically significant increasing orthophosphate-concentrations trends in groundwater for the early time period except for the Lincoln County HZ and the step-trend for the Minidoka County HZ. Orthophosphate concentration trend rates for the early period were low, ranging from 0.001 to 0.015 mg/L/year. Only two HZs and the well networks had enough orthophosphate concentration data available in the latter time period to do statistical analysis. The two HZs (Minidoka and Southern Gooding/Jerome County) both have decreasing orthophosphate concentration trends, with only the monotonic trend for the Southern Gooding/Jerome County HZ being statistically significant at 90 percent with a rate of −0.001 mg/L/year.
Groundwater levels in two well networks in the eastern Snake River Plain aquifer were also evaluated for trends (monotonic and step), with both networks having statistically significant declining groundwater levels for the 1993–2009 trend period. The latter trend period (2010–20) had statistically significant declining groundwater levels for the A&B well network and statistically significant increasing groundwater levels for the Jerome/Gooding well network, which is downgradient from an aquifer recharge area.
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This study was funded in cooperation with the Idaho Department of Environmental Quality and the Middle Snake Regional Water Resources Commission. I am especially thankful to the following people for providing nitrate, orthophosphate, or groundwater-level data: Amy Steimke and Paul Thomas (Idaho Department of Water Resources); Kathryn Elliott (Idaho Department of Environmental Quality); and Curtis Cooper (Idaho Department of Agriculture).
Multiply | By | To obtain |
Length | ||
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foot (ft) | 0.3048 | meter (m) |
Area | ||
acre | 4,047 | square meter (m2) |
acre | 0.4047 | hectare (ha) |
acre | 0.4047 | square hectometer (hm2) |
acre | 0.004047 | square kilometer (km2) |
square foot (ft2) | 929.0 | square centimeter (cm2) |
square foot (ft2) | 0.09290 | square meter (m2) |
square inch (in2) | 6.452 | square centimeter (cm2) |
section (640 acres or 1 square mile) | 259.0 | square hectometer (hm2) |
square mile (mi2) | 259.0 | hectare (ha) |
square mile (mi2) | 2.590 | square kilometer (km2) |
Multiply | By | To obtain |
Volume | ||
---|---|---|
liter (L) | 33.81402 | ounce, fluid (fl. oz) |
liter (L) | 2.113 | pint (pt) |
liter (L) | 1.057 | quart (qt) |
liter (L) | 0.2642 | gallon (gal) |
Mass | ||
gram (g) | 0.03527 | ounce, avoirdupois (oz) |
Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).
Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
Concentrations of chemical constituents in water are given in milligrams per liter (mg/L). Milligrams per liter is equivalent to parts per million (ppm).
Altitude, as used in this report, refers to distance above the vertical datum.
Environmental Data Management System
eastern Snake River Plain
Idaho Department of Environmental Quality
Idaho Water Resource Board
Nitrate Priority Area
National Water Information System
National Water Quality Program
U.S. Geological Survey
Groundwater quantity and quality are ongoing concerns in Idaho. Groundwater supplies most of the state’s drinking water: almost 85 percent in 1990, almost 95 percent in 1995, and almost 89 percent in 2015 (
Location of study area, hydrogeologic zones, and two well networks in Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties in south-central Idaho, referred to as the “Mid-Snake” area.
Figure 1. Map showing location of study area, hydrogeologic zones, and two well networks in Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties in south-central Idaho, referred to as the “Mid-Snake” area
Land use in the middle portion of the ESRP aquifer (Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties in south-central Idaho, referred to hereinafter as “Mid-Snake”) (
Previous studies conducted in the Mid-Snake detected elevated concentrations of nitrate in several public- and domestic-use wells (
The Idaho Department of Environmental Quality (IDEQ) has identified groundwater areas within Idaho that have elevated nitrate concentrations in groundwater. The nitrate-degraded groundwater areas are referred to as “Nitrate Priority Areas” (NPA) and are listed based on the severity of the degradation (
This report documents concentration trends over time of nitrate + nitrite as nitrogen and orthophosphate as phosphorus for two periods in groundwater within six hydrogeologic zones (HZs) in the Mid-Snake region of south-central Idaho and two 30-well well networks that overlap with two of the HZs. Two types of trends are evaluated: monotonic and step. The monotonic trends are a constantly changing trend over time, either increasing or decreasing. A step trend represents a change of values between two points in time, either an increase or a decrease. A step trend does not account for any change between time periods, only the beginning and end of a given time period. Where possible, the HZs are defined to coincide with county boundaries to aid local managers with applying trends results to local management policies. Nitrate concentration data are defined as “nitrate + nitrite as nitrogen from a filtered sample of groundwater” henceforth referred to as nitrate. Nitrite analyses from the data sources described below indicate that most nitrite concentrations are below lab detection levels so nitrate + nitrite as nitrogen concentrations represent nitrate concentrations. The orthophosphate data is defined as “orthophosphate as phosphorus from filtered samples of groundwater” henceforth referred to as orthophosphate.
Groundwater-level trends were evaluated for two periods (1993–2009 and 2010–20) in the two well networks using the Regional Kendall statistical test (
The study area is approximately 7,800 square miles (mi2) (
The ESRP regional basalt aquifer, which encompasses most of the study area, is primarily vesicular and fractured olivine basalt flows (Quaternary age) of the Snake River Group (
Groundwater in the tributary valley aquifers south of the Snake River moves northward and recharges the ESRP aquifer or discharges to the Snake River. The tributary valley aquifers are predominantly alluvial deposits that interfinger with the basalts of the Snake River Group near the mouths of the valleys. A more complete description of these valley aquifers is provided by
A lack of nutrient-concentration data prevents temporal trends from being evaluated at individual wells. Grouping nutrient-concentration data together from multiple wells in an area or zone provides enough data to do the statistical tests and hence evaluate temporal trends or changes in nutrient concentrations. These HZs (
The HZs are similar to one another but do have some differences. The Cassia and Twin Falls Counties HZs contain tributary valley aquifers with groundwater moving northward to the ESRP or discharge directly to Snake River as described above. These two HZs coincide the most to county boundaries. The Minidoka County HZ contains the ESRP aquifer and the Burley Perched aquifer with the Burley Perched aquifer being excluded from this study. The Minidoka County HZ follows the Minidoka County boundary on the north, east, and southern sides. The western boundary extends beyond the Minidoka County boundary following the groundwater flow direction to include Jerome County agricultural land that impacts nitrate and orthophosphate concentrations. The Jerome/Southern Gooding County HZ is also composed of the ESRP aquifer. The boundaries follow regional groundwater-flow paths through the study area from the northeast to the southwest and discharge via springs along the Snake River. The Lincoln County HZ is composed of the ESRP aquifer and is predominantly recharged by tributary valleys from the north. It differs from the Jerome and Minidoka County HZs which receive regional groundwater flow from the upper portion of the ESRP. The northern Gooding County HZ is downgradient of the Lincoln County HZ but has more agricultural land use.
Groundwater levels (expressed as altitude above the land datum) from two 30-well networks that are part of the USGS National Water Quality Program (NWQP) were analyzed for trends. The two well networks were established in 1993 (A&B well network) and 1994 (Jerome/Gooding well network) and had groundwater levels measured by the NWQP in June during various years in the 1990s and on an alternately biennial basis starting in the early 2000s with data available from
Nitrate and orthophosphate concentration trends were evaluated for each HZ in the study area and for the two well networks. Groundwater-quality data were compiled from the USGS National Water Information System (NWIS) (
As noted above, the nitrate concentration data used are defined as nitrate + nitrite as nitrogen and referred to as just “nitrate.” The orthophosphate concentration data used are specifically orthophosphate as phosphorus and just referred to as “orthophosphate.” The compiled nitrate and orthophosphate data were processed so that the concentrations were expressed in the units described, and duplicate results were excluded from the dataset.
Nitrate and orthophosphate concentration data were limited to samples collected during May, June, July, or August to control seasonal variability. The resultant nitrate concentration dataset contains 4,663 concentrations from groundwater sampled from 515 wells, with the majority in June (58 percent) and July (30 percent). The orthophosphate concentration dataset contains 780 concentrations from 228 wells, with most in June (40 percent) and then July (29 percent) and August (29 percent) (
Location of recent concentrations of nitrate and orthophosphate in Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties in south-central Idaho, referred to as the “Mid-Snake” area. Data from
Figure 2. Map showing location of recent concentrations of nitrate and orthophosphate in Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties in south-central Idaho, referred to as the “Mid-Snake” area
Regional statistical tests or statistical tests involving data from multiple wells are necessary because there are not enough nitrate and orthophosphate concentration data available to evaluate trends at individual wells. The Regional Kendall and Wilcoxon signed rank statistical tests are ideal for evaluating trends amongst many wells in an area. The main difference between the two tests is that the Regional Kendall test evaluates all available data at each well for the entire time period providing a monotonic trend result or a trend that consistently increases or decreases through time. The Wilcoxon signed rank statistical test evaluates a step trend or change between paired results from groups of wells in an area (
The Regional Kendall test (
Wells with only two samples cannot determine monotonic temporal trends; they can only infer a step trend or change between two time periods. In these cases, the Wilcoxon signed rank test is preferred (
The significance level of 90-percent confidence (p-value less than or equal to 0.10) means the data have a 90-percent probability of a trend occurring. The 90-percent confidence was selected instead of a 95-percent confidence to provide a more conservative indication of changes in concentrations and to allow early warning in those cases that may not have been significant at the 95-percent confidence. Whether or not the trend is hydrologically occurring or hydrologically important is not defined by a p-value alone. It is possible a trend may exist but be at such a low level that it is not hydrologically important. To address the importance of a trend, the number of samples should be included, as well as the magnitude or size of the trend (
Nitrate and orthophosphate concentration data were compiled from many different sources as discussed above with differing detection levels, typically reported with a “<” concentration level or “ND” (non-detection). Comparing non-detection concentrations at different concentration levels may result in false trends (
Groundwater-level trends were evaluated for the two NWQP well networks (
Table 1. Groundwater-level trends for two well networks in the Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties in south-central Idaho, referred to as the “Mid-Snake” area
[Data taken from
Well network | Time period | p-value | Monotonic |
Trend |
Regional Kendall trend test | ||||
---|---|---|---|---|
A&B well network | 1993–2009 | <0.01 | −0.58 | Decreasing |
A&B well network | 2010–20 | <0.01 | −0.72 | Decreasing |
Jerome/Gooding well network | 1994–2009 | <0.01 | −0.26 | Decreasing |
Jerome/Gooding well network | 2010–19 | 0.06 | 0.14 | Increasing |
Wilcoxon signed rank test | ||||
A&B well network | 1993–2009 | <0.01 | −0.79 | Decreasing |
A&B well network | 2010–20 | <0.01 | −0.52 | Decreasing |
Jerome/Gooding well network | 1994–2009 | <0.01 | −0.24 | Decreasing |
Jerome/Gooding well network | 2010–19 | <0.01 | 0.35 | Increasing |
Groundwater-level monotonic trends (Regional Kendall test) and step trends (Wilcoxon signed rank test) of 90 percent confidence in two well networks in south-central Idaho. Data taken from
Figure 3. Map showing groundwater-level monotonic trends and step trends of 90 percent confidence in two well networks in south-central Idaho
Two well networks and hydrographs of groundwater levels in selected wells, south-central Idaho. Data taken from
Figure 4. Map showing two well networks and hydrographs of groundwater levels in selected wells, south-central Idaho
Nitrate and orthophosphate concentrations in groundwater were evaluated for trends in the two NWQP well networks and the six HZs (
The NWQP well networks reside within the Minidoka County and Southern Gooding/Jerome County HZs. However, the NWQP well networks were evaluated separately from the HZ networks to compare the well networks with previous NWQP evaluations. Wells in both the A&B and Jerome/Gooding NWQP well networks were sampled in 2005, and then a subset of seven wells from each well network were sampled biennially from 2006 to 2012. The A&B and Jerome/Gooding well networks were resampled entirely again in 2016 and 2017, respectively. Some wells from both networks were sampled by other projects or agencies; these nitrate and orthophosphate concentrations were included in the statistical trends analysis. Statistical comparisons for the A&B and Jerome/Gooding well networks cover only the time periods of 2002–19 and 2000–19, respectively, because most concentration data were collected in 2005, 2016, and 2017.
Nitrate concentration trends in both the A&B and Jerome/Gooding groundwater well networks are increasing at a rate of 0.3 mg/L per year except for the A&B well network early period step trend (Wilcoxon signed rank test), which indicates a nitrate concentration trend that is increasing at a lower rate (0.02 mg/L per year) and is not statistically significant at p ≤0.10 (
Table 2. Nitrate and orthophosphate concentration monotonic trends at 90 percent confidence for two time periods in the Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties in south-central Idaho, referred to as the “Mid-Snake” area
[Data taken from
Monotonic trends (Regional Kendall test) | ||||||
Hydrogeographic zone, |
Time period | Concentration |
Number |
p-value | Monotonic |
Monotonic trend result |
A&B - NWQP, nitrate | 2002–19 | 89 | 21 | <0.01 | 0.03 | Increasing trend |
A&B - NWQP, orthophosphate | 2002–17 | 81 | 20 | <0.01 | 0.001 | Increasing trend |
Jerome/Gooding - NWQP, nitrate | 2000–19 | 96 | 26 | <0.01 | 0.03 | Increasing trend |
Jerome/Gooding - NWQP, orthophosphate | 2000–19 | 91 | 26 | <0.01 | 0.001 | Increasing trend |
Cassia County, nitrate | 2000–09 | 70 | 28 | 1.00 | NA | Trend not statistically significant |
2010–19 | 76 | 32 | 0.09 | 0.06 | Increasing trend | |
Cassia County, orthophosphate | 2000–09 | 66 | 26 | <0.01 | 0.001 | Increasing Trend |
2010–19 | 2 | 1 | NA | NA | Insufficient data | |
Northern Gooding County, nitrate | 2000–10 | 28 | 10 | 0.01 | 0.55 | Increasing trend |
2011–20 | 50 | 19 | 1.00 | 0.00 | Trend not statistically significant | |
Northern Gooding County, orthophosphate | 2000–10 | 28 | 10 | <0.01 | 0.015 | Increasing trend |
2011–20 | 0 | 0 | NA | NA | Insufficient data | |
Jerome / Southern Gooding County, nitrate | 2000–09 | 98 | 31 | 0.07 | 0.01 | Increasing trend |
2010–20 | 35 | 35 | 0.24 | 0.02 | Trend not statistically significant | |
Jerome / Southern Gooding County, orthophosphate | 2000–09 | 94 | 31 | <0.01 | 0.002 | Increasing trend |
2010–20 | 43 | 16 | 0.06 | −0.001 | Decreasing trend | |
Lincoln County, nitrate | 2000–10 | 32 | 12 | 0.62 | 0.01 | Trend not statistically significant |
2011–20 | 28 | 14 | 0.23 | 0.01 | Trend not statistically significant | |
Lincoln County, orthophosphate | 2000–09 | 30 | 11 | 0.30 | 0.002 | Trend not statistically significant |
2011 and 2019 | 2 | 1 | NA | NA | Insufficient data | |
Minidoka County, nitrate | 2000–10 | 1,806 | 170 | <0.01 | 0.01 | Increasing trend |
2011–20 | 1,809 | 189 | <0.01 | 0.09 | Increasing trend | |
Minidoka County, orthophosphate | 2000–09 | 93 | 26 | 0.06 | 0.011 | Increasing trend |
2010–18 | 89 | 9 | 0.16 | −0.001 | Trend not statistically significant | |
Twin Falls County, nitrate | 2000–09 | 22 | 10 | 0.37 | −0.02 | Trend not statistically significant |
2010–19 | 157 | 66 | 0.24 | 0.02 | Trend not statistically significant | |
Twin Falls County, orthophosphate | 2000–09 | 118 | 42 | <0.01 | 0.002 | Increasing trend |
2010–19 | 2 | 1 | NA | NA | Insufficient data |
Table 3. Nitrate and orthophosphate concentration step trends at 90 percent confidence for two time periods in Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties in south-central Idaho, referred to as the “Mid-Snake” area
[Data taken from
Step trends (Wilcoxon signed rank test) | ||||
Hydrogeographic zone | Time period | p-value | Step trend rate |
Step trend result |
A&B - NWQP, nitrate | 2002–19 | 0.12 | 0.02 | Trend not statistically significant |
A&B - NWQP, orthophosphate | 2002–17 | <0.01 | 0.024 | Increasing trend |
Jerome/Gooding - NWQP, nitrate | 2000–19 | 0.03 | 0.03 | Increasing trend |
Jerome/Gooding - NWQP, orthophosphate | 2000–19 | 0.06 | 0.001 | Increasing trend |
Cassia County, nitrate | 2000–09 | 0.09 | 0.12 | Increasing trend |
2010–19 | 0.18 | 0.26 | Trend not statistically significant | |
Cassia County, orthophosphate | 2000–09 | <0.01 | 0.012 | Increasing trend |
2010–19 | NA | NA | Insufficient data | |
Northern Gooding County, nitrate | 2000–10 | 0.01 | 0.28 | Increasing trend |
2011–20 | 0.38 | 0.72 | trend not statistically significant | |
Northern Gooding County, orthophosphate | 2000–10 | 0.02 | 0.015 | Increasing trend |
2011–20 | NA | NA | Insufficient data | |
Jerome / Southern Gooding County, nitrate | 2000–09 | 0.14 | 0.06 | Trend not statistically significant |
2010–20 | 0.06 | 0.12 | Increasing trend | |
Jerome / Southern Gooding County, orthophosphate | 2000–09 | <0.01 | 0.009 | Increasing trend |
2010–19 | 0.63 | −0.006 | Trend not statistically significant | |
Lincoln County, nitrate | 2000–10 | 0.28 | 0.05 | Trend not statistically significant |
2011–20 | 0.20 | 0.05 | Trend not statistically significant | |
Lincoln County, orthophosphate | 2000–09 | 0.13 | 0.127 | Trend not statistically significant |
2011 and 2019 | NA | NA | Insufficient data | |
Minidoka County, nitrate | 2000–09 | <0.01 | −0.22 | Decreasing trend |
2010–19 | <0.01 | 0.75 | Increasing trend | |
Minidoka County, orthophosphate | 2000–09 | 0.40 | 0.045 | Trend not statistically significant |
2010–18 | 0.19 | −0.001 | Trend not statistically significant | |
Twin Falls County, nitrate | 2000–09 | 0.81 | −0.03 | Trend not statistically significant |
2010–19 | 0.24 | 0.16 | Trend not statistically significant | |
Twin Falls County, orthophosphate | 2000–09 | <0.01 | 0.001 | Increasing trend |
2010–19 | NA | NA | Insufficient data |
Nitrate concentration monotonic trends (Regional Kendall test) and step trends (Wilcoxon signed rank test) at 90 percent confidence for two well networks and hydrogeologic zones for two time periods along with Nitrate Priority Areas in south-central Idaho. Data taken from
Figure 5. Map showing nitrate concentration monotonic trends and step trends at 90 percent confidence for two well networks and hydrogeologic zones for two time periods along with Nitrate Priority Areas in south-central Idaho
Orthophosphate concentration monotonic trends (Regional Kendall test) and step trends (Wilcoxon signed rank test) at 90 percent confidence for two well networks and hydrogeologic zones for two time periods in south-central Idaho. Data taken from
Figure 6. Map showing orthophosphate concentration monotonic trends and step trends at 90 percent confidence for two well networks and hydrogeologic zones for two time periods in south-central Idaho
The two statistical tests for trends (monotonic and step trends) of nitrate concentrations in groundwater produced differing results in the Cassia County HZ. The Regional Kendall test, which determines a monotonic trend, indicates no trend for the early time period (2000–09) and a statistically significant increasing trend of 0.06 mg/L per year for the latter time period (2010–19). The step trend (Wilcoxon signed rank statistical test) is the opposite in that it has a statistically significant increasing nitrate concentration trend for the early time period but not the latter time period. The early time period has a statistically significant trend of 0.12 mg/L per year and the latter time period a trend of 0.26 mg/L per year but not statistically significant at p ≤ 0.10 (
Orthophosphate concentration trends are increasing in the Cassia County HZ according to both statistical tests for the early time period (2000–09), although at different rates. The increasing monotonic trend determined by the Regional Kendall test has a statistically significant rate of 0.001 mg/L per year while the step trend statistically significant increasing rate determined by the Wilcoxon signed rank test is 0.01 mg/L per year (
Groundwater in the northern Gooding County HZ has statistically significant increasing nitrate concentration monotonic and step trends for the early time period (2000–10). The Regional Kendall test indicates the highest median trend rate of any HZ at 0.55 mg/L per year. However, the Wilcoxon signed rank test estimates the step-trend rate at 0.28 mg/L per year (
Orthophosphate concentrations have statistically significant increasing trends for both tests during the 2000–10 time period with median trend rates of 0.015 mg/L for the Regional Kendall test and 0.001 mg/L per year for the Wilcoxon signed rank test (
Groundwater in the Jerome/Southern Gooding County HZ has a statistically significant increasing nitrate concentration monotonic trend for the 2000–09 time period at a low median trend rate of 0.01 mg/L per year. The step trend also has an increasing nitrate concentration for the same time period however it is not statistically significant at 90 percent confidence (
Orthophosphate concentrations have increasing trends for both the monotonic and step trend tests with rates of 0.002 and 0.009 mg/L per year from 2000 to 2009 (
Groundwater in the Lincoln County HZ indicate increasing monotonic and step trends for nitrate concentrations however none of the trends are statistically significant. This is the case for both trend types and time periods for nitrate concentrations. The same is true for the early time period (2000–10) for orthophosphate concentrations–an increasing trend but not statistically significant (
The Minidoka County HZ is the only HZ with opposing nitrate-concentration monotonic and step trend results. The Regional Kendall test indicates a statistically significant increasing nitrate concentrations monotonic trend of 0.01 mg/L per year for the 2000–10 period while the Wilcoxon signed rank test indicates a statistically significant decreasing step-trend rate of −0.22 mg/L per year for the same period (
The latter trend period (2011–20) has statistically significant increasing nitrate concentration trends for both the monotonic and step trends. However, the trend rates are very different, with the monotonic trend rate at 0.09 mg/L per year and the step-trend rate at 0.75 mg/L per year. This is another example of how concentration data during the middle of the time period affects the trend results, similar to the nitrate concentration trend during the previous time period.
Orthophosphate concentrations in the Minidoka County HZ have increasing trends for the early time period (2000–09). However, only the monotonic trend is statistically significant, with an increasing orthophosphate concentration rate of 0.011 mg/L per year (
Groundwater nitrate concentration trends in the Twin Falls County HZ are not statistically significant for either trend period (2000–09 or 2010–19) (
Orthophosphate concentration trends in the Twin Falls County HZ are increasing at a rate of 0.002 (monotonic trend) and 0.001 (step trend) mg/L per year during the 2000–09 period (
Nitrate and orthophosphate concentration trends in groundwater were evaluated for two time periods in the Middle Snake River Region in south-central Idaho. Two trend-evaluation methods were used: the Regional Kendall test (monotonic trends) and the Wilcoxon signed rank test (step trends). The findings from the present study complement other studies that have conducted similar trend evaluations overlapping spatially and temporally with this study. Because most of the other studies only considered nitrate concentration trends and not orthophosphate concentration trends, the comparison discussions only cover the nitrate concentration trends. The Idaho Department of Environmental Quality (IDEQ) has conducted nitrate concentration change evaluations for nitrate concentrations measured from 2002–06 and 2007–11 (
Step-trend comparisons between this study and the IDEQ study (
Nitrate concentration step trends for the HZ and NPA areas are compared by
The U.S. Geological Survey National Water Quality Program has been measuring groundwater levels in two 30-well networks in the eastern Snake River Plain aquifer since 1993. These groundwater levels have been declining during that time until 2016, when they started to rise. The Idaho Water Resource Board began aquifer recharge in the area starting in 2013, with increased aquifer recharge starting in 2017 and continuing through 2019. While groundwater levels appear to be rising or at least no longer declining, there has been no analysis to confirm these findings. Nitrate and orthophosphate concentrations in groundwater, like groundwater levels, have been changing over time with no recent studies to assess nitrate concentration trends and no studies at all evaluating orthophosphate concentration trends.
This report evaluates groundwater level and nitrate and orthophosphate concentration trends in the middle Snake River (mid-Snake) area (Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties) in south-central Idaho. Two types of temporal trends were evaluated: monotonic and step. The monotonic trends are a constantly changing trend over time, either increasing or decreasing and determined using the Regional Kendall statistical test. A step trend represents a change of values between two times, either an increase or decrease change and determined with the Wilcoxon signed rank statistical test. The temporal trends were evaluated in specific regions defined as HZs within the Mid-Snake area. The HZs were defined as areas of similar geologic and hydrologic characteristics. The HZ boundaries were chosen to coincide with county boundaries, where possible, so that temporal nutrient concentration trend evaluations would provide the most benefit to local water managers.
The A&B and Jerome/Gooding well networks both had statistically significant declining groundwater level trends for the early time period (1993/94–2009) but only the A&B well network had declining groundwater level trends during the latter time period (2010–20). The Jerome/Gooding well network, which is downgradient from the IWRB aquifer recharge area, had statistically significant increasing groundwater level trends for the 2010–19 time period. The groundwater-level trends in the A&B well network vary from about 0.5 to 0.8 feet drop per year for the entire time period (1993–2020). The Jerome/Gooding well network had dropping groundwater levels at a rate of about 0.25 feet/per year from 1994–2009 and then the groundwater levels rose at a rate of 0.14–0.35 feet/per year from 2010–20. Both well networks show rising groundwater levels starting in 2016 but only the Jerome/Gooding well network groundwater levels increased enough to overcome 2010–15 declines.
Nitrate and orthophosphate concentration trends for groundwater in HZs in the mid-Snake area varied by hydrogeologic zone and trend time period with most of the trends increasing and about half having a statistically significant increasing trend. Nitrate concentrations in groundwater have statistically significant increasing trends (monotonic and step) for the early time period (2000–10) in the Northern Gooding HZ. The Jerome/Southern Gooding County HZ also has increasing nitrate concentration trends with only the monotonic trend being statistically significant. The two trend types differ in results for the Cassia County and Minidoka HZs. For the Cassia County HZ, there is a statistically significant increasing step trend but no monotonic trend, and the Minidoka County HZ differs with statistically significant increasing monotonic trend and decreasing step trend. The Twin Falls and Lincoln County HZs do not indicate nitrate concentration trends for either type of trend or either time period.
For the latter time period (2011–20), only the Minidoka County HZ has statistically significant increasing nitrate concentration for both trend types. Cassia and Jerome/Southern Gooding County HZs have increasing trends for both monotonic and step trends but only one of each are statistically significant. The Twin Falls, Lincoln, and Northern Gooding County HZs do not have nitrate concentration trends for the latter time period.
Orthophosphate concentrations in groundwater have statistically significant increasing trends for the early period in four of the HZs (Twin Falls, Cassia, Jerome/Southern Gooding, and Northern Gooding County). Minidoka County also has increasing trends, but only the monotonic trend is statistically significant. The Lincoln County HZ does not have orthophosphate concentration trends for the early time period. Orthophosphate concentrations in the latter time period are decreasing in the Minidoka and Jerome/Southern Gooding County HZs, but only the monotonic trend in the Jerome/Southern Gooding County HZ is statistically significant. The other four HZs (Twin Falls, Cassia, Lincoln, and Northern Gooding Counties) do not have enough orthophosphate concentration data available for the statistical tests.
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Science Publishing Network, Tacoma Publishing Service Center
For more information concerning the research in this report, contact the
Director, Idaho Water Science Center
U.S. Geological Survey
230 Collins Rd
Boise, Idaho 83702-4520