2909 17-Mile Drive © 2000 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

July 27, 2000

Basin Watch

by

Leo Poppoff

A NEW LOOK AT WATERSHED EROSION AND WATER QUALITY

TRPA asked a consultant, Hydro Science, to review how wetlands affect
water quality in the Tahoe Basin. The resulting report is stimulating a
debate over the value of erosion control in the fight to stem Lake
Tahoe's decline.

Wetlands are valuable because they can strip nutrients from stormwater.
In the Tahoe Basin, we expect wetlands to clean up a particular form of
nutrients -- the form that's readily used by algae. Nitrogen, phosphorus
and iron compounds that stimulate algal growth are said to be
"bioavailable."

To get a handle on the kind of job wetlands are expected to do in the
Tahoe Basin, Hydro Science researchers had to identify sources of
bioavailable nutrients, and get an idea of how much flows into the lake.
There isn't a lot of data on this subject -- but there is some. And it
led the report's author to an intriguing conclusion.

First, let's review some basics. Bioavailable forms of nutrients are
dissolved, but not all dissolved forms are bioavailable. For example,
nitrate and ammonia are dissolved forms of inorganic nitrogen that algae
sop up. But, there are organic nitrogen compounds (from vegetation and
animals) that are also dissolved. According to the report, studies
elsewhere show that very high percentages of organic nitrogen compounds
in runoff are dissolved.

Algae can't use these organic nitrogen compounds. But with time, some of
the dissolved organic nitrogen can be converted to ammonia or nitrates
by microbes. And so can particulate forms of organic nitrogen, but not
as easily. The extent of conversion of organic nitrogen in the Tahoe
Basin isn't known, so it's hard to get a handle on just how much
nitrogen is bioavailable.

Phosphorus is intimately tied up with soils and rocks. The principal
source of phosphorus in Tahoe soils is the mineral apatite, according to
Hydro Science researchers. Two dissolved forms of phosphate stimulate
algal growth, but they're usually lumped together and called
orthophosphates.

Dissolved nitrogen flows with water. It doesn't attach to soils, though
it can be taken up by plants. So when nitrogen is in a sediment laden
stream or infiltrates into the ground, it just keeps going, eventually
ending up in the lake.

Dissolved phosphorus, on the other hand, latches onto the first soil
particle it sees. So, when it's infiltrated, it doesn't flow well with
ground water and when it enters a turbid stream, its concentration
diminishes. The affinity of orthophosphate for soil is so great,
according to Hydro Science, only a small percentage of phosphate applied
as fertilizer is actually available for plants.

Orthophosphate comprises 6 to 19 percent of the phosphorus loading to
the lake. How much of the particulate phosphate is eventually converted
to orthophosphate is not known. But, preliminary studies seem to show
that it's small. That answer might change with more intensive research.

Complicating the picture even more, is the possible role of fine
particles, called silt and clay. Tiny particles are suspended in the
lake for a long time because they settle to the bottom very slowly.
These particles scatter light, and help diminish clarity.

Because the particles hang around for a comparatively long time,
attached orthophosphate might eventually let go and become snack food
for Tahoe's algae. It's also been suggested that small organic particles
floating in the lake might eventually be converted to bioavailable
nitrogen.

Nearly half of all particulate phosphorus is associated with course soil
particles, according to the Hydro Science report. Soils in the Tahoe
Basin are course-textured with a low content of fine particles. Just 3.4
percent of the Tahoe Basin contains soils with greater than 50 percent
silt and clay. Particles getting in the lake fall to the bottom and are
buried, so phosphorous then becomes unavailable.

Much of the bioavailable nitrogen is brought from both distant and local
sources by the atmosphere and falls directly on the lake. We've given up
on trying to control algal growth by limiting nitrogen because it
appears to be out of our control. So, we're concentrating on phosphorus.

Bioavailable orthophosphate comes from the watershed, mostly from
urbanized areas. And it seems that much of the fine-particle load of
phosphorus also comes from the atmosphere -- but from closer sources,
like Tahoe's roads.

Much of our effort in the basin has been to control erosion in order to
prevent bioavailable nutrients from getting to the lake. The techniques
we use are most effective at controlling relatively course-textured
soils. Yet, dissolved bioavailable nutrients, as well as very fine
particles, are not trapped -- even though it seems that they are
responsible for loss of clarity in the lake.

So, where is all this leading us? Stay tuned.