September 2006, Vol. 18, No.9
Research Notes - Beachgoers Now Have Another Worry: Polluted Sand
It’s a warm, sunny day, and the beach is calling. Beware, however, say researchers from the University of California–Los Angeles (UCLA). Even when surrounding ocean waters test clean, the sand on the beaches may be laced with harmful levels of bacteria, especially in the sheltered areas favored by parents with toddlers.
The researchers found fecal indicator bacteria, such as Escherichia coli and enterococci, to be prevalent in the top layer of sand at some of Southern California’s most popular beaches in the Santa Monica Bay area, according to a report (in press) in the journal Water Research. At some of the enclosed beaches studied, enterococci levels were roughly 1000 times higher than levels observed at beaches open to pounding ocean surf, said Jennifer Jay, an environmental engineer at UCLA and the report’s corresponding author.
State water quality officials routinely use such indicator bacteria to determine the water quality at recreational beaches. High levels, which are most often due to stormwater runoff and leaky wastewater collection systems, according to the U.S. Environmental Protection Agency, typically lead to beach postings. The microbial quality of beach sand, however, is continuously overlooked, Jay pointed out.
“What is startling about our findings is that even when the water shows low bacteria levels, there are still high levels of bacteria that persist in the sand,” Jay said.
At the exposed beaches, the researchers found that levels in sediments peaked along with — but didn’t persist significantly longer than — concentrations in the water column during a storm. At an enclosed beach, however, they documented high levels in sediments throughout the study period.
Others have shown that the high-energy waves at open beaches increase resuspension and outward transport of sediments and microbial contaminants, so the finding that indicator bacteria didn’t persist in these sediments wasn’t a surprise, Jay said. The study showed, however, that low-energy waves at enclosed beaches didn’t have nearly as significant a dilution factor in the surf zone. Additionally, the more stationary environment may promote more favorable conditions for fecal indicator bacteria to survive and grow, as UCLA laboratory analyses indicated.
How much of a human health risk these findings might connote is difficult to say, Jay admitted, because no health standards exist for beach sediments as they do for the water column. And while other studies have correlated high fecal indicator bacteria levels in water with human illnesses, no studies to date have done so with sediments.
“It’s just a route of exposure that people don’t think about, or they may have looked into it at open beaches and thought it probably doesn’t matter,” Jay said. At the same time, “I know as a parent that children often put things in their mouths, including sand, and that concerns me.”
Jay cautioned, though, that fecal indicator bacteria may behave differently from their pathogen counterparts in sediments. “The survival of these indicator organisms in sand points to the persistence of other disease-causing organisms in sand, which could be very significant,” she said. “But we don’t yet have enough data to know how significant.” Jay and her colleagues are now focusing on the persistence of viruses in beach sediments.
Sri Lanka Water Supply Still Suffers Effects of 2004 Tsunami
Sri Lanka’s coastal drinking water supply continues to suffer the effects of the December 2004 tsunami, which caused major death and destruction in the region, according to paper published in the May issue of the American Geophysical Union (AGU; Washington, D.C.) journal Water Resources Research. The report was prepared by a 14-member international team of scientists and engineers from the United States, Sri Lanka, and Denmark.
Much of Sri Lanka’s coastal area relies on wells, which usually are hand-dug and relatively shallow, according to an AGU press release. About 40,000 such wells, each serving several families, were destroyed or contaminated by the tsunami. The continued sustainability of the aquifers that supply such wells is questionable, due to continued saltwater contamination, erosion of beaches, and other human impacts, such as sand mining, increased pumping, and pollution, according to the press release.
The tsunami, which reached up to 1.5 km (0.9 mi) inland, poured seawater, along with other contaminants, directly into the open dug wells, making those that were not destroyed unusable. In some areas, as many as four large tsunami waves struck, with the second in the series often the largest. Aside from contamination of wells, large quantities of seawater penetrated from the flooded surface of the land through porous layers below and into the aquifer, the press release says.
Efforts to restore wells by pumping out seawater were sometimes counterproductive, researchers found, as excessive pumping may have allowed more seawater to enter the aquifer from below. This pumping also caused many wells to collapse, as their walls were not reinforced. Finally, contaminated water that was pumped out of wells was often discharged in places that permitted contaminants to seep back into the aquifer and again into the wells.
The researchers, led by professor Tissa Illangasekare of the Center for Experimental Study of Subsurface Environmental Processes at the Colorado School of Mines (Golden), found that one anticipated consequence of the disaster did not materialize — waterborne disease — thanks to public awareness of the need to disinfect wells and practice good personal and food hygiene.
Although some of the affected coastal aquifers in Sri Lanka are composed of ancient limestone deposits, most coastal groundwater is stored in sandy aquifers that are replenished by rainwater, especially during the monsoon season from October to February. This recharge has been slow in many of the most affected areas, as they did not receive substantial rainfall for almost a year, the press release notes.
Researchers say it will take several more monsoon seasons for the aquifers to recover. In collaboration with the American, Danish, and Sri Lankan scientists, a group of researchers at the International Water Management Institute based in Sri Lanka is conducting long-term monitoring studies at selected sites.
The researchers say that since March 2005, salinity levels have declined slowly, if at all, in many of the wells that continued to be pumped. They note that planning is under way to provide piped water to many coastal villages to supplant the individual, and vulnerable, open dug wells. Other social responses include plans for expanding centralized sewage collection, proposed setbacks for housing along coastlines, and the use of new modeling techniques for integrated management of surface water and groundwater for sustainable water resources.
Such floods can be caused by more than tsunamis, the researchers note, including storm surges, hurricanes or cyclones, and rising sea level. They urge hydrologists to participate in developing emergency planning procedures that could greatly reduce human suffering. Documenting the hydrologic impacts of such disasters is, they say, the first step toward developing internationally recognized emergency guidelines for treating sources of contaminated water supplies and for long-term and planning tools for managing coastal groundwater in areas affected by seawater inundation.
The team has developed a number of recommendations, which they will present to the Sri Lankan government, to help develop local expertise and capacity-building in areas of modeling, data management, and subsurface characterization for integrated water management in the affected regions. The authors say they are continuing to work together to address Sri Lanka’s water needs.
Desalination Roadmap Seeks Technological Solutions To Increase Water Supply
A rising global population and limited sources of fresh water have researchers and scientists scrambling to find ways to make fresh water available for consumption. Sandia National Laboratories (Albuquerque, N.M., and Livermore, Calif.) may have found an alternative source for potable water.
Sandia researchers Pat Brady and Tom Hinkebein are putting the final touches on the updated Desalination and Water Purification Roadmap — Roadmap 2 — that should result in more fresh water in parts of the world where potable water is scarce. The roadmap will recommend specific areas of potential water desalination research and development that may lead to technological solutions to water shortage problems, according to a Sandia news release. Roadmap 2 will outline the specific research needed in high-impact areas to create more fresh water from currently undrinkable brackish water, from seawater, and from wastewater.
“There will be 29% more of us in 20 years,” said Hinkebein, manager of Sandia’s geochemistry department and head of Sandia’s Advanced Concepts Desalination Group. “Put that together with an unequal distribution of people — more moving to Texas, California, Arizona, and New Mexico where fresh water is limited — and it is easy to see we are facing a challenging water future.”
According to the news release, only 0.5% of Earth’s water is directly suitable for human consumption. The rest is composed of saltwater or locked up in glaciers and icecaps. The increased water demand will have to be fulfilled by something, and brackish water seems to be a natural source, Hinkebein explained.
Following completion of the second roadmap, the Joint Water Reuse and Desalination Task Force will then submit it to U.S. Sen. Pete Domenici (R–N.M.), chairman of the Senate Energy and Water Development Appropriations Subcommittee, then to the U.S. Congress and eventually the water user and research communities. The task force consists of the U.S. Bureau of Reclamation (Washington, D.C.), the WaterReuse Foundation (Alexandria, Va.), the American Water Works Association Research Foundation (Denver), and Sandia.
“The task force will decide which of the 43 projects get to the top of the research pile,” Brady said. “As more money is made available, universities, research groups, national laboratories and private companies will bid on projects.”
There are 43 research areas that have been identified for Roadmap 2, including membrane technologies (mainly reverse osmosis) that desalinate and purify water by pushing it through a semipermeable membrane that removes contaminants; alternative technologies that take advantage of nontraditional methods; concentrate-management technologies that consider the disposal and beneficial use of desalination wastestreams; and reuse and recycling technologies that look at ways membrane and alternative technologies will be used to more efficiently recycle water.
Slippery Nanotubes Open Door to Cheaper Desalination
Defying classical fluid dynamics, scientists have shown experimentally that carbon nanotubes allow a relatively fast flow of water and gas to pass through their cores. The discovery could mean huge energy savings for desalination processes.
“The water and gas flows that we measured are 100 to 10,000 times faster than what classical models predict,” said Olgica Bakajin, a physicist at Lawrence Livermore National Laboratory and corresponding author of research published in the May 19 issue of Science. “This is like having a garden hose that can deliver as much water in the same amount of time as a fire hose that is 10 times larger.”
On a silicon chip the size of a quarter, the researchers fashioned a membrane made of carbon nanotubes. These tubes, less than 2 nm in diameter, are hollow cylinders 50,000 times thinner than a human hair, Bakajin said. Billions of them, aligned in a unique arrangement, act as the pores in a membrane.
Despite their smaller inner diameter, these nanotubes allow liquids and gases to flow through rapidly, while the tiny pore size blocks larger molecules, Bakajin noted. This effect “makes them very efficient filters of any sort, and desalination is one big application that we’re targeting,” she said.
Reverse osmosis (RO) is the current state-of-the-art method for desalination, but its high-energy costs have hampered its applicability. Because the nanotubes have a much higher permeability than RO membranes, “you’re going to get more water with the same amount of pressure applied across the membrane,” Bakajin explained. As a result, “you’re going to get more energy-efficient production of water,” with energy costs up to 75% less, compared to RO membranes.
Exactly why the nanotube membrane works so well is still an open question, according to Bakajin. “It’s a unique nanoscale effect, but there’s still work to be done on really understanding the mechanisms,” she said. “Since water doesn’t wet the outside surface of carbon nanotubes, we were skeptical that water would enter them, let alone flow really fast.”
What Bakajin and her colleagues suspect is that small molecules may be traveling quickly through the carbon nanotubes, in part, because the tubes have slippery walls. In other words, “gas and water molecules may be bouncing off the nanotube’s atomically smooth luminal surface like billiard balls,” Bakajin said.
Another potential application for the carbon nanotube membrane, with its high permeability and affinity for hydrocarbons, is in industrial gas separation processes. “We may be able to use them for the cost-effective removal of greenhouse gases from power plant flue gas, which would have a profound effect on the environment,” Bakajin added.