2909 17-Mile Drive © 2000 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

August 10, 2000

Basin Watch

by

Leo Poppoff

TIME TO RETHINK CHERISHED NOTIONS?

Can wetlands, retention basins, infiltration trenches, or other erosion
control devices mitigate damage done by urban development? Can they
clean runoff water before it gets to Lake Tahoe? We act as if they can.
But, do we know that they'll do the trick -- or are we just hoping?

TRPA, to its credit, recently asked the same question, and hired a
consultant to review the facts. The consultant, Toby Hanes of Hydro
Science, was formerly a U.S. Forest Service hydrologist in the Tahoe
Basin and has since worked on wetlands restoration projects around the
lake. His report makes some unsettling observations on the effectiveness
of mitigation measures.

Infiltration trenches are potentially valuable tools for eliminating
dissolved phosphorus. The dissolved portion of phosphorus, the phosphate
ion, is the form that stimulates algal growth. And, fortunately, it's
attracted to soil particles, where it sticks tightly.

Yet, as Hanes explains in his report, phosphate ions are attracted to
some soils, like fine clays, more than to coarser soils, like decomposed
granite. Phosphate ions are cleaned out of water faster by fine silts
and clay than by coarser decomposed granite. So, ideal conditions for
removing dissolved phosphorus from runoff are lots of fine soils and
long contact times.

But, infiltration trenches in the Tahoe basin are relatively narrow, and
much of our soil is very permeable decomposed granite. This means that
runoff water can move pretty quickly through the trenches and into
ground water. So, for most infiltration trenches in the basin,
conditions are not ideal for removing phosphorus. And perhaps that's one
reason why dissolved phosphorus is found in ground water, which
eventually flows to the lake.

Sedimentation, or retention, basins are intended to catch dirty runoff
and hold it until sediments settle out. Vegetation planted in these
basins should use the nutrients in runoff, and thereby cleanse dissolved
nitrogen and phosphorus from the water. But, as Hanes points out, that
process takes time. And most Tahoe Basin retention basins aren't large
enough to hold storm runoff or spring snowmelt long enough to work their
magic.

Wetlands, marshes and meadows, are nature's ultimate retention basins.
They slow the water, so sediment falls out. They have well-established
vegetation, soils and nitrogen chomping microbes. Still, their
usefulness in the Tahoe Basin is also in question, according to Hanes.

Runoff must be stored for a while for all the natural processes to work.
In some meadows, water is channeled, flows rapidly through, and isn't
there long enough for fine soils to settle out or for vegetation to suck
up nutrients. Smaller marshes suffer similar limitations.

But, at high altitudes with cold winter conditions, another problem
occurs. The largest flows of runoff occur in fall, winter and early
spring -- before plants start growing. If runoff water caught by
wetlands isn't retained until the warmer growing season, not much
nutrient is likely to be removed from it. And during the cold season,
microbes that transform nitrates into harmless, gaseous nitrogen and
nitrous oxide aren't very active. Incidentally, nitrous oxide is
harmless for the lake, but it's a greenhouse gas.

To be useful, wetlands should be near urbanized areas. Yet, except for
sections of South Shore, wetlands aren't located where they can treat
urban runoff. As they say in the real estate business, "it's location,
location, location."

But wouldn't it be still be useful to treat the non-urbanized 80 percent
of the watershed that could run into streams through riparian vegetation
and into the lake through meadows and marshes? Wouldn't that do a lot
for us? Well, the little information we have might make us wonder if
that's the place to put our money.

Hanes presents data showing that runoff from urban areas has fourteen
times as much dissolved phosphorus as from non-urbanized areas.
Dissolved nitrogen in the form of ammonia is eleven times higher in
urban runoff. And in nitrate form, it's three times as plentiful.

So, it seems to be most important \-- and most difficult \-- to treat
urban runoff. Hanes points to the possibility that controlling
commercial fertilizer application and sewer line leaks in the basin
could cut nitrate and phosphate contamination of groundwater by half.
But, the largest source of nutrients going to the lake is from the
atmosphere -- accounting for 48 percent of dissolved nitrogen and 33
percent of dissolved phosphorus. This part of the nutrient budget can't
be treated by wetlands, retention basins or infiltration trenches.

Hanes, and others, are limited in making predictions about the
effectiveness of our nutrient control program because there is very
little scientifically valid information. Huge gaps exist in our
knowledge of nutrient sources and how to control them. Even so, Hanes
hopes that his report stimulates discussion about the important issue of
how to save Lake Tahoe.

Next time, we'll finish the provocative Hydro Science report.