Number range CHAPTER© 2004 by Leo Poppoff

December 16, 2004

Basin Watch

by

Leo Poppoff

HELPING COLD CREEK BY INTERCEPTING POLLUTANTS

Back in the 1960s, when subdivisions were sprouting like mushrooms
around the Tahoe Basin, some developers had what they thought was a
great idea. They dammed Cold Creek to make Lake Christopher. Cold Creek
is near South Shore. It's the largest watershed tributary to Trout
Creek, which in turn is the second largest tributary to Lake Tahoe. Cold
Creek once meandered in a shallow streambed through a beautiful lush
meadow. During high flows, the stream overflowed and its water filtered
through the meadow.

Lake Christopher undoubtedly helped sell houses in that neighborhood.
Still, to environmentalists, it was a disaster. A beautiful meadow was
drowned and, later, Cold Creek was diverted into a straight ditch and
around the man-made lake.

In 1994, the dam was removed. Cold Creek was restored to a meandering
stream, and the meadow revived. It's now a beautiful scene, and the
meanders lessen the chance of downstream flooding. But, the stream
channel is deeper than it was a half century ago, and has more meanders.

The creek's channel was designed to contain all but extreme flows, so it
was dug deeper than the original stream. As a consequence, the streambed
was deep enough to intercept ground water. And, more important, runoff
from nearby subdivisions and Pioneer Trail was sending pollutants
directly into the creek.

El Dorado County and the California Tahoe Conservancy teamed up to build
a retention basin to intercept the polluted runoff and treat it.
Researchers with the U.S. Geological Survey (USGS) instrumented the
basin to learn how it worked. USGS folks especially wanted to know how
ground water that flows through and under the retention basin affects
the basin's effectiveness and the quality of stream water.

The site of the retention basin is just off Pioneer Trail, at the end of
Cattleman's Court. It's where excavated soils were dumped during
construction of nearby homes. The basin was dug out of the fill, which
is four to five feet thick and covers natural meadow soils.

David Prudic, USGS hydrologist, leads the research team. He explains
that meadow soils are rich in organic materials and sediments that were
laid down as Cold Creek overflowed its banks. This has occurred for
centuries and even millennia.

The important aspect of this is that the organic material is
decomposing, and the products include dissolved nutrients, such as
nitrogen and phosphorus. Beneath the meadow, subterranean flows carry
those nutrients as well as dissolved minerals. So, even without the
additions from urban runoff, ground water naturally carries some
nutrients and minerals. The mix and amounts depend on the kinds of soils
it runs through. What the USGS folks were interested in was the
interaction of water trapped in the retention basin with the ground
water flowing under the basin.

Prudic points out that water held in the basin settles down into the
ground water, adding to its nutrient and mineral content. So in 2000,
the USGS drilled 30 monitoring wells throughout the basin before its
construction, and installed sampling equipment in storm drains. Water
from wells, storm drains, and from Cold Creek above and below the
retention basin, was routinely sampled before and for three years after
construction of the basin.

What they found, according to Prudic, was that the basin effectively
keeps sediment from the urban runoff from entering into Cold Creek. The
basin hardly ever overflows. When it does, flows are directed away from
the creek and sediments are trapped by meadow vegetation.

What about nutrients added by urban runoff? Prudic and his colleagues
found that the runoff water retained in the basin did not noticeably
change the nutrient or mineral content of the ground water. More
importantly, there was no evidence of increased nutrient or mineral
content in Cold Creek. Why not is testimony to the power of soil
bacteria.

Populations of the kind of bacteria that decompose nutrients likely
increased in the ground water immediately beneath the retention basin.
Prudic suggests that the bacteria were so stimulated by the influx of
oxygen in the runoff water that they processed the extra nutrients. Even
so, bacterial action is slower in soils deeper beneath the basin. That's
because organic decomposition in the soil under the retention basin uses
up oxygen, which some types of soil bacteria also need.

On the other hand, USGS researchers have observed that there was a steep
drop in nutrient concentrations between monitoring wells and the waters
of Cold Creek, just three or four feet away. Prudic believes that this
was caused by soil bacteria getting oxygen from the mixing of Cold Creek
water in soils beneath the streambed, and being much more effective in
removing nutrients in that short stretch.

So, the bottom line is that the retention basin cleans sediment from
urban runoff. And, nutrients in the runoff are not adding to natural
nutrient levels of the ground water leaving the basin. This research is
providing valuable insights into the action of this kind of retention
basin. The final report, due out next winter, should be very useful for
retention basin designers.

Comments or questions? Send them to <basinwatch@sbcglobal.net>