Stable Isotope Recharge Project
The Department of Water and Resource Conservation (Department) will conduct a recharge study using stable isotope analyses to better understand the mixing of recharge sources and contributions of local precipitation and river water to the groundwater basin as sampled from wells, particularly in the area of Butte Creek. This study seeks to answer questions of how water moves into and through the system in this area, what are the primary sources of recharge and how do they change spatially and with depth.
The recharge project is based on initial work and a recommendation of future work that came from the Lower Tuscan Aquifer Project (LTA Project). The LTA Project involved an evaluation of chemical constituents (i.e. stable isotopes) of ground and surface water. As an add-on, samples were taken from several locations, which proved to be valuable in gaining insights on the identification and elimination of potential major recharge areas. The results from the LTA Project were promising, but require additional investigation to further our understanding of basin recharge.
One of the recommendations in Future Data Needs from the LTA Project is an expansion of stable isotope analysis to include greater spatial coverage, explore the seasonal variation in the stable isotope signature of hydrogen and oxygen, and consider use of stable isotopes of other elements including possibly boron, carbon, sulfur, and/or nitrogen.
Stable Isotopes Explained
Stable isotope analysis of water samples is based on the principal that the hydrogen and oxygen atoms that form water molecules contain different isotopic forms. Isotopes are two forms of the same element that have different numbers of neutrons and thus differ in atomic mass. However, isotopes have the same chemical properties. Stable isotopes are those that do not undergo radioactive decay and, thus, do not change composition over time. The most prevalent stable isotopes in water are deuterium (D) and oxygen-18 (18O). These isotopes are present at very low levels in seawater, in the parts per thousand range and have atomic masses that are greater than those of the most common isotopes, which are hydrogen (H) and oxygen-16 (16O).
The proportion of deuterium and 18O in a water sample is reported relative to a standard value, defined as Standard Mean Ocean Water (SMOW) on a parts per thousand (per mil) basis. As water vapor moves inland from the ocean and forms clouds, the isotopic composition of the resulting precipitation is affected by the temperature, altitude, and distance from the ocean. Precipitation that falls at lower latitudes, higher temperatures, and/or lower altitudes will tend to have a higher proportion of the heavier isotopes compared to precipitation that falls farther to the north, farther inland, and at higher altitudes. As the proportion of heavier isotopes in the precipitation decreases with increasing distance from the ocean, for example, the water is said to be more depleted. This effect is illustrated by the cartoon to the left. In California, the isotopic shift in precipitation owing to cloud movement inland from the coast has been documented.
Isotope data are reported using delta notation (δ) and are displayed graphically with 18O (δ18O) on the x (horizontal) axis and deuterium (δD) on the y (vertical) axis. Because the isotopic data are presented as ratios relative to the SMOW isotope concentrations, the δ18O and δD values are negative numbers. Thus, data values that plot farther to the left and lower on the chart represent precipitation that fell farther from the ocean and/or at higher elevations than data values that plot farther to the right and higher on the chart, as illustrated on the graph. This is useful because different water sources can have distinct isotopic signatures and therefore this can be used to better understand regional recharge and groundwater flow paths at falls farther to the north, farther inland, and at higher altitudes. As the proportion of heavier isotopes in the precipitation decreases with increasing distance from the ocean, for example, the water is said to be more depleted. This effect is illustrated by the cartoon to the left. In California, the isotopic shift in precipitation owing to cloud movement inland from the coast has been documented.
Recharge Study Objectives
On May 1st, 2015 the Department began the process to solicit proposals to conduct a study that will help us to better understand the dynamics of recharge on the perimeter of the basin as geology and soil characteristics transition from the lower foothills into the valley floor. The desired outcomes of the project include the development of the following:
- A comprehensive compilation of available stable isotope and other relevant water quality data from previous studies throughout the Northern Sacramento Valley (Sutter Buttes to Redding). This will include a summary of pertinent conclusions and interpretations of recharge processes from this data;
- An updated conceptual model of source waters, recharge processes, and basin dynamics south of Chico and in the Butte Creek area;
- Analysis and a map that quantifies relative amounts of water source (local precipitation versus stream recharge) in sampled groundwater;
- A greater understanding of vertical groundwater movement and recharge sources with depth by sampling groundwater from multi-completion well nests;
- A greater understanding of seasonal variability of stable isotopes in sampled surface and groundwater;
- Specific recommendations for how results should influence and inform mapping of recharge areas in the county, and;
- A public outreach and education program that will heighten public awareness and understanding of the aquifer and recharge processes.