Dr. Vance Kennedy is a long time champion of farmers and farmland. Along with Wood Colony’s Jake Wenger and Modesto’s Denny Jackman, Dr. Kennedy is sponsoring an urban limits initiative that would protect prime farmland around Modesto. Like many Valley citizens, Dr. Kennedy is alarmed at intensive groundwater mining in the foothills of eastern Stanislaus County. His background as an award-winning hydrologist enables him to offer authoritative analysis of the perils brought about by mining groundwater for unsustainable farming practices. Here, in his own words, are Dr. Kennedy’s latest comments on mining groundwater in the foothills.
Farmers in the foothills have three sources of water: rain (12 to 16 inches per year), groundwater directly below their property (occupies perhaps 15 percent of pore space in the rocks), and underground flow from aquifers underlying adjacent properties, including those having reservoirs or rivers on them.
Nut trees require at least 36 inches of water per year to grow well. At most, 6 inches of that water comes from rain because of runoff and evapotranspiration by other plants.
The various sources of water can be identified by differences in their stable isotope composition.
As storms move up the mountains, the heavier isotopes rain out preferentially, so high elevation snow and rainfall contain a greater proportion of light isotopes. Hence, when that snow melts and fills the reservoirs, that high-elevation water can be identified by isotopic analysis. Rivers draining those reservoirs will also be identifiable.
Rain falling directly on the foothills will have a significantly greater level of heavy isotopes than that in the reservoirs and rivers. Hence, the two sources can be distinguished in the foothill groundwater and the relative quantities of the two sources estimated.
Wells pumping within a few thousand feet of rivers and reservoirs will cause the water table to drop and increase infiltration from the rivers and reservoirs, thereby reducing the quantity of water flowing in the rivers supplying the irrigation systems in the Valley. People who have surface water rights should be outraged by the “theft” of their water. There should be rules preventing this underground “stealing.” The rules do not exist now, but can be passed by local supervisors.
When rain falls it contains radioactive tritium with a relatively short “half-life.” Thus, tritium becomes an age-dating tool. Pumped groundwater can be age dated. Hence, pumped water can not only be identified as to source but the length of time it has been below ground can be estimated.
A combination of groundwater modeling and isotropic analysis would go a long way to determine where the water is coming from that is supplying trees planted on rangeland in the foothills. People relying on river water in the Valley need to learn the extent to which water use in the foothills is affecting their water supply and act accordingly. It can be done using well recognized tools and an understanding of groundwater hydrology. It need not be unreasonably expensive. The only thing preventing the knowledge is lack of action by the powers that be and political pressure by wealthy persons benefitting from the present situation.
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