September 14, 2016 | Water in the West | Insights
What does your Sonoma County Chardonnay have in common with your Paso Robles Syrah? One likely answer is groundwater. It takes about 75 gallons of water to grow grapes for one gallon of wine in the California North Coast region. Much more is needed in arid places like Paso Robles or the Central Valley.
Groundwater’s importance to California extends far beyond wine grapes; it provides up to 60 percent of the total water supply in a dry year. But after decades of groundwater reliance and insufficient recharge of water back into the ground, many such basins around California are overtapped. It doesn’t help that groundwater is inherently hard to see, measure and understand. Passage of the Sustainable Groundwater Management Act (SGMA) in 2014 provided a framework to manage groundwater in California by requiring newly designated agencies to create plans to guide each groundwater basin to sustainability. To implement SGMA, the California Department of Water Resources (DWR) recently released emergency regulations that will help local agencies shape the plans.
But creating a good plan is only the start. Monitoring is key to a successful outcome. One basic and crucial piece of information that must be gathered is groundwater elevations, or levels. Knowing if aquifer elevations are increasing, decreasing or stable, and where this is happening, alerts managers to the potential for a variety of problems and highlights where management efforts need to be focused. It is hard to overstate how fundamentally important groundwater levels data are to understanding aquifer health.
DWR’s draft regulations underscore the importance of groundwater levels by calling for “a sufficient density of monitoring wells capable of collecting representative measurements through depth discrete perforated intervals to adequately characterize the potentiometric surface for each of the principal aquifers.” This basically means having enough monitoring wells for each main aquifer in order to determine groundwater levels. Understanding the health of each of the different aquifers in a system requires measuring each aquifer separately. This is what a dedicated monitoring well does: it is targeted and screened to a specific aquifer.
While simple in concept, developing a robust system of monitoring wells is not easy or cheap. Dedicated monitoring wells are expensive: one could easily run between $100,000 to $200,000. Many groundwater basins in California have multiple aquifers at depth, which means that developing a dedicated monitoring well requires drilling separate wells “screened” or perforated to measure conditions for each aquifer.
Factors such as geology, casing, well depth, and borehole diameter affect costs. It is also more expensive to drill deeper wells: for example, the Water Replenishment District of Southern California’s ballpark estimates for drilling monitoring wells are $70 to $100 per foot for a well around 100 feet or less, $150 to $200 per foot for a 100 to 500 foot well, and $250 to $350 per foot for anything over 500 feet. Other expenses such as land acquisition and permitting could increase the total significantly.
Given the expense of dedicated monitoring wells, many agencies cannot afford to have as many as they need to cover their management area. To address data gaps, many water agencies use production wells, or those developed for water supply, to monitor groundwater. Domestic production wells such as those drilled for household use typically access one shallow aquifer. Many larger municipal production wells access multiple aquifers in order to pump more water at once.
Recently released results from a survey of investigated local groundwater data collection practices conducted by Tara Moran and others at Stanford University found that monitoring wells account for only about a quarter of the wells used in groundwater monitoring networks, with the majority of monitoring done using production wells. Of concern is the fact that 12 percent of monitoring networks do not have a single dedicated monitoring well.
While production wells can be a useful component in a groundwater monitoring program, results from these wells have their limitations and may be misleading. Because production wells are intended to maximize water production, they can be screened across multiple aquifers. Measured groundwater levels (or water quality data, for that matter) could then be indicative of conditions across all the aquifers, rather than of a single aquifer. Production wells that are screened across multiple aquifers are most useful for monitoring when their data can be correlated with nearby dedicated monitoring wells.
On an even more basic level, sometimes production wells are missing basic information such as well depth or geology to indicate what aquifers the well is tapped into. Data from these wells are typically qualified, such as being assigned a lower weight when used in groundwater models. Or they may be discarded when more accuracy is desired.
While the gold standard for obtaining aquifer-specific groundwater levels is through the use of targeted, short-screened dedicated monitoring wells, production wells are already a vital (or the only) part of many local water agencies’ monitoring programs. Given that many water agencies currently have very limited information about their groundwater systems, it makes sense to start with any information they can get, such as data from production wells, while prioritizing investment in dedicated monitoring wells. Developing a comprehensive groundwater monitoring network will take time; however, resources such as grant funding from the state will help expedite this process.