In Search of the Source of Beach Pollution, Scientists Monitor Groundwater

New Sea Grant Study to Look at Northern California Beaches

February 17, 2006

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

With NOAA support, two Stanford researchers will soon test a theory that may explain chronic beach pollution in California and elsewhere.

The wet sand in the photo is submarine groundwater seeping out during a very low tide.
Photo: Alexandria Boehm, Stanford University

The theory: groundwater discharging to the coast is indirectly or directly increasing the abundance of harmful bacteria – the kind that close beaches to swimming – and may be as important to understanding coastal pollution as the more often implicated urban runoff.

The research, to begin in March, is a follow-up on field experiments conducted in 2003 at Huntington Beach, Calif. – an area well known for a mysterious rash of summer-time beach closures. In this work, Alexandria Boehm, a professor of environmental engineering, and Adina Paytan, a professor of geological and environmental sciences, showed that the amount of submarine groundwater flowing into the surf zone varies with the phase of the moon and that microbial pollution levels are higher when a lot of submarine ground water is making it to the beach.

Groundwater is water from rain, snowmelt, irrigation, etc. that seeps underground, filling pore spaces of rocks and sediments. Unless trapped, groundwater eventually flows to sea. Along the way, it may become contaminated with bacteria and nutrients from leaky sewers, septic tanks, lawn fertilizers, pet waste and the like. “Submarine groundwater” refers to the underground mix of groundwater and saltwater that ebbs and flows into coastal waters with the pull of the tides.

Although researchers have shown that bacterial counts vary with lunar cycles at 50 Southern California beaches, they have yet to show what would be expected to be true – that groundwater itself is a source of bacteria. In fact, only 1 of 26 groundwater samples tested at Huntington Beach had elevated levels of fecal indicator bacteria, the standard by which authorities regulate beach water quality.

The central question is to explain how bacterial counts could be so closely linked to submarine groundwater discharges if the groundwater itself is not contaminated with bacteria?

Researchers collected about 11 metric tons of beach water with trashcans, allowing them to test for the relatively low concentrations of radium, a widely accepted tracer of groundwater.
Photo: Alexandria Boehm, Stanford University

The new NOAA California Sea Grant project will shed light on the question and allow the scientists to investigate possible explanations.

A leading hypothesis, Boehm says, is that beach sand may store fecal indicator bacteria. Groundwater may "free" bacteria that would otherwise remain held in sediments.

Another possibility: groundwater contains dissolved organic matter and nutrients - phosphate, nitrate and ammonia - that encourage the growth of fecal indicator bacteria.

In their 2003 study, scientists found that nitrate concentrations were 100 times higher in submarine groundwater than in the surf zone. Nitrogen and phosphate might influence the growth of plankton and could influence the occurrence of toxic algal blooms in some areas, Boehm said.

Stanford professor Alexandria Boehm in the light blue jacket, on the far right, and graduate students who helped gather field data. The long white pole is a “well point” ¬– a long hollow tube through which groundwater is extracted.
Photo: Alexandria Boehm, Stanford University

Another issue to be explored is the extent to which land-use patterns influence submarine groundwater and beach water quality. During their Sea Grant project, Boehm and Paytan will collect water samples from beaches in Santa Cruz, which sits within NOAA's Monterey Bay National Marine Sanctuary and Bolinas, which is north of San Francisco and within the Gulf of the Farallones National Marine Sanctuary.

These sites have different population densities and wastewater management plans. Residents of rural Bolinas are on septic tanks, which means waste is released into the ground. Santa Cruz, in contrast, is relatively urban, and the city’s treated sewage is discharged at sea.

"We are the only researchers studying the connection between submarine groundwater discharges and California coastal water quality," Boehm said. "The Sea Grant research will allow us to document how the quantity and quality of submarine groundwater affects beach water quality. ... There are many regulations for what is discharged from the land to the sea via runoff. Results from our work may suggest that what is discharged from the land to the sea via the subsurface should be regulated, too."