Geologic And Hydrologic Factors Governing ... - Gunnison County

Geologic And Hydrologic Factors Governing Impacts Of Development
On The Crystal River Near Marble, Colorado
Gunnison County, Colorado
Because nitrate is expected to be the limiting pollutant with respect to groundwater contamination,
an understanding of nitrate concentrations typically associated with septic system effluent as well as
nitrate’s behavior in the unsaturated and saturated zones is important.
The total nitrogen in septic tank effluent, which is routed to the leaching field has a concentration of
40 mg/L (Canter and Knox 1985). Most of the nitrogen is converted to nitrate-nitrogen, which is
referred to as nitrate.
Nitrate is very soluble and, therefore, mobile. It moves freely with groundwater without retardation.
Therefore, the primary means of reducing nitrate concentrations is by dilution.
The 40 cfs winter (10/1-4/30) in-stream flow decree for the Crystal River downstream of Carbonate
Creek and the 45 cfs year-around decree for the short reach upstream of Carbonate Creek to the con-
fluence provides enormous dilution potential. As will be shown later, the impact of a large number
of properly installed leaching fields in the Marble area would not substantially degrade the water
quality of the surface flow of the mainstream due to nitrate loading. Other pollutants, such as patho-
gens, could have an adverse effect on its water quality. The groundwater in the Crystal River allu-
vial aquifer, however, is more susceptible to pollution than is the surface flow.
The tributaries of Slate Creek and Carbonate Creek are a different matter, however. These two
creeks are subject to the same antidegradation rules as those which apply to the Crystal River.
As will be discussed later, the pollution potential of Slate Creek and Carbonate Creek is more severe
from the standpoint of mountainside development.
5.9 Typical Flows And Nitrate Loads
A typical Equivalent Residential Unit (EQR) for the purposes of this water quality management
study only is defined as:
1. Single Family Residence (SFR);
2. Two Bedrooms;
951-110.000 Wright Water Engineers, Inc. Page 50