CRITICAL LOADS FOR INORGANIC NITROGEN
DEPOSITION IN THE COLORADO FRONT RANGE, USA
Mark W Williams
Department of Geography and
Institute of Arctic and Alpine Research
University of Colorado, Boulder
Air Resources Division
National Park Service
We suggest an empirical approach for determining critical loads
for inorganic N deposition in wetfall to the central Rocky Mountains.
We define "critical loads" as a deposition amount above which
natural resources can be negatively affected.
The arithmetic average from 1992 to 1996 of annual inorganic N
deposition in wetfall at the eight NADP sites located
than 2,500\|m in the central Rocky
Mountains ranged from
2.5 to 3.5\|@kghayr@.
In contrast, inorganic N deposition was less than
2.5\|@kghayr@ at all 23 NADP sites
less than 2,500\|m in elevation.
At the Niwot Ridge NADP site in the Colorado Front Range,
a simple linear regression of inorganic N in wetfall
with time shows a
significant increase in deposition of inorganic N in wetfall at
the rate of
(@r2@ = 0.62; p < 0.001, n = 13).
the increasing amount of inorganic N in wetfall
is causing episodic acidification in headwater catchments
of the Green Lakes Valley in the Colorado Front Range,
with acid neutralizing capacity (ANC) values below 0\|@mueq@ in surface
waters during snowmelt runoff at 9-ha and 42-ha sampling sites.
At present rates of ANC decrease,
we can expect the 9-ha and 42-ha sites to become
chronically acidified within the next decade
and the 220-ha basin of Green Lake 4 to become
A synoptic survey in 1995 of 91 high-elevation lakes in the central Rocky
Mountains suggests that water quality is being affected by inorganic N in wetfall
throughout the region.
Federal land managers are required to "err on the side of protection" when
assessing the amount of deposition that will alter ecosystem processes.
However, given the political and economic ramifications of policy decisions,
land managers are aware of the need to provide a scientific basis for these
decisions and to balance conflicting needs.
To achieve this balance and to
allow for natural resource protection, we make a conservative
that critical loads of inorganic N in wetfall to Class 1 areas in in the central
Rocky Mountains be set at 4\|@kghayr@.
Target loads my be set at lower levels of
inorganic N deposition in wetfall
to allow a margin of safety to protect extremely sensitive natural