Drained island pump stations adjacent to levee walls. Photo courtesy of Christina Richardson.
Groundwater pumped from subsided islands back into Delta channels through 200 or more active outfalls can be laden with excess nitrogen transported from island soils, report the authors of a new study.
The Sacramento-San Joaquin Delta contains more than 50 peat islands. Most have subsided—some as much as three meters — as a result of drainage for farming, and must maintain artificial water tables below the land surface via managed pumping. According to researchers
from the University of California at Santa Cruz and the US Geological Survey, these pump stations are an underappreciated source of nutrients in nearby waters, with potentially significant implications for habitat and water quality.
To get a sense of how much nitrogen is leaving the islands and when, the researchers measured monthly discharge along with nutrient and trace element concentrations in drainage from three Delta islands over the course of nearly a year and a half, then extrapolated to the wider system. Their findings, reported in San Francisco Estuary and Watershed Science, suggest that with recent upgrades to the Sacramento River Wastewater Treatment Plant, previously the largest source of ammonium inputs to Delta waters, these drained islands may now be the dominant source. Additionally, existing Delta nutrient “budgets,” which widely figure nitrogen inputs from island drainage are negligible may need to be reconsidered.
The study’s results are preliminary and must be refined through higher-resolution measurements and estimates, says lead author Christina Richardson of UC Santa Cruz. But the findings align with other work
by the same group investigating carbon exports from drained Delta islands, and appear robust enough to at least challenge current assumptions. “Island drains have been overlooked as sources,” Richardson says.