When water is scarce, urban rivers can help supply meet demand

There are a lot of thirsty customers for the world’s fresh surface water – in the United States, for example, 64 percent of all the water used in 2000 came from lakes, rivers, and reservoirs. But here’s the problem: it’s very rare. Only eight thousandths of a percent (.008%) of the world’s water is fresh surface water. With such high demand and limited supply, regional and municipal water authorities must be very creative and careful about how to satisfy users ranging from fish to farmers to families.

Enter Stanford civil and environmental engineering Professor Martin Reinhard, whose research on reusing water is helping water districts in Northern and Southern California make the most of their local water supplies in the nation’s largest and thirstiest state.

Frequently, fresh water sources are drawn upon so heavily, they cannot be replenished naturally. Rather than allow water levels to dwindle, districts are beginning to consider pumping treated wastewater into local river systems as a means of restoring local drinking water supplies, bolstering threatened riparian habitats, or satisfying other uses. The practice of mixing wastewater into rivers has inspired some public concerns and with health and ecosystems on the line, it certainly does have to be done right. But what Reinhard and students are finding, as they monitor river flows and concentrations of contaminants such as drugs and detergents, is that the practice may indeed be a very good idea.

“There is an opportunity for reuse, especially in the dry states [like] the American southwest, where typically rivers don’t flow at all unless there is some sort of wastewater feeding it,” Reinhard says. “What we are arguing for, in a very cautious way, is that with reuse in rivers one can actually ‘polish,’ or help clean, the water and at the same time make it publicly more acceptable as part of an overall treatment process.” Reinhard’s interest in water reuse has led him to two California water districts, one in Orange County south of Los Angeles and one closer to home in Santa Clara County. In the course of his work, he is generating a wellspring of knowledge about the ability of urban rivers to actually purify their water.

Rivers clean their water

The Santa Ana River in Orange County is an example of a river that has so many demands on it that it would barely be flowing in the dry summer months if upstream communities didn’t discharge highly treated wastewater into it. Aware of the potential risk this posed for groundwater supplies, the local water district sought to study it carefully.

What perhaps could have been worrisome turned out to be beneficial. After years of studying contamination levels in the water, what Reinhard and students have found is that the river system actually has a great capacity to clean up water as it flows through. The findings should help local residents feel more at ease, he says.

“The surprise of the findings is that the river was not given credit as a treatment system in itself,” Reinhard says. “People were thinking that what goes in comes out. But it removes a lot of contaminants. It has an innate self–purification process.” Not for all contaminants, of course, but for many.

For example, in a study of an 11 kilometer stretch of the Santa Ana, Reinhard and his students found that concentrations of the anti–cholesterol drug gemfibrozil dropped by two thirds and the presence of a form of the pain killer ibuprofen vanished from the water completely. In another similar study, Reinhard and his team saw the concentration of another common pain–killer, naproxen, drop by about 80 percent.

River systems accomplish this self–purification through a variety of means. Some contaminants end up becoming absorbed into the system’s sediment, rocks, or even aquatic plants (water that penetrates all the way down to aquifers is filtered further as it goes). Sometimes sunlight striking the water breaks down the chemical structure of contaminants, or catalyzes a reaction between the contaminant and another chemical. Reinhard found the reduction in concentrations of naproxen was greater during the day than at night, for example. In other cases, bacteria and other organisms in the water can remove contaminants.

The ability of rivers to clean their water gives them a potential role in water management. In an article in the May 2006 issue of the journal Environmental Science and Technology Reinhard and student Christopher Gurr suggest that managers consider actively using rivers as one step in a thorough multi–step treatment process. Water would become progressively purer as it passes through treatment plants, river systems and the earth as it percolates into aquifers. Doing this has the potential to increase locally available water supplies by making wastewater suitable for reuse.

A solution to scarcity?

The issue is now on the table in the Santa Clara Valley Water District where sustaining the habitat of San Jose’s Coyote Creek is a high priority, but so is meeting heavy demand for the water stored in the Anderson Reservoir, which feeds the creek. The solution may well be to release treated wastewater into the creek to compensate for water diverted from the reservoir.

To study this, Reinhard and students Gurr and Megan Plumlee have set up a research program designed to gather data on the safety of that idea for humans and wildlife. Every river is different, after all, and what seems to be working well in the Santa Ana river, may or may not work well in Coyote Creek. “There are a number of precautions that have to be taken,” Reinhard says.

One precaution is, as in Orange County, to determine whether contaminants would percolate through to groundwater supplies, which are used for drinking. Another precaution is ensuring that the water is safe for river wildlife. After all, the whole concept of the project is to help sustain the habitat.

In his research, therefore, Reinhard has three tasks. One will be to use harmless chemical tracers to track where they end up when they are added to the creek (i.e. further downstream, or in groundwater). Another will be to monitor contaminant levels as experimental amounts of wastewater are added to the creek, to see whether they approach harmful concentrations and whether the concentrations diminish as the river self–purifies. A third will be to determine whether the creek remains at the appropriate temperature for protected steelhead trout to thrive.

Reinhard says officials will take their time to make sure the project is safe. “This will be done in a very cautious fashion in very small increments,” he says.

But if all goes well, water reuse in Coyote Creek could help the district meet increasing demand, without having to sacrifice a vital urban habitat, much as water reuse seems to be safely recharging the Santa Ana river system and nearby groundwater supplies. As demand continues to increase in many regions of the world where water is scarce, reuse involving rivers may become more necessary. In their May 2006 paper, Reinhard and Gurr write: “Indirect potable reuse is a solution to water scarcity that increasingly makes sense for many regions of the world. With careful research and far–sighted policy–making, the sustainable use of water resources and a closed water loop are within reach.”

Martin Reinhard, professor of civil and environmental engineering at Stanford University