Stage 1 Report
First Quarterly Progress Report
Final Report Page 66 October 2004
atrazine TMDLs are warranted. The discussion of sources contributing to low
dissolved oxygen, manganese, pH and atrazine in Skillet Fork Segment CA 03 also
applies to Segment CA 05, with two exceptions: algal respiration and ammonia
nitrification are not thought to be significant factors contributing to low dissolved
oxygen in this segment (CA 05). Because of the naturally high levels of manganese in
the soils, the general use and public water supply criteria may be difficult to attain.
• For Skillet Fork (Segment CA 06), data are considered sufficient to support the
causes listed on the draft 2004 303(d) list, and dissolved oxygen, manganese, pH, and
atrazine TMDLs are warranted. The discussion of sources contributing to low
dissolved oxygen, manganese, pH and atrazine in Skillet Fork Segment CA 03 also
applies to Segment CA 06. Because of the naturally high levels of manganese in the
soils, the general use criterion may be difficult to attain.
• For Skillet Fork (Segment CA 09), data are considered sufficient to support the
cause listed on the draft 2004 303(d) list, and a dissolved oxygen TMDL is warranted.
It should be noted that the listing, while warranted, is based on only three dissolved
oxygen measurements, with two of the three measurements being approximately 2
mg/l below the criteria of 5 mg/l. Causes of low dissolved oxygen typically include
sediment oxygen demand, degradation of CBOD, nitrification of ammonia and/or
algal respiration. Although there were insufficient data to assess the relationship
between ammonia and dissolved oxygen, SOD and DO, CBOD and DO, and
chlorophyll and DO, potential sources in this watershed were identified through the
watershed characterization. These sources may include runoff from agricultural land
(fertilized cropland and agricultural land with livestock) and failing private sewage
disposal systems (lesser extent). There are no point sources in this watershed.
Similar to what was observed farther downstream of this segment, low flows and high
temperatures may also exacerbate low dissolved oxygen concentrations in this
segment.
• For Horse Creek (Segment CAN 01), data are considered sufficient to support the
listing of this segment for dissolved oxygen; however, the data were extremely
limited. Only two of the four measurements were below the DO criterion. The data
are also sufficient to support the listing of this segment for manganese with one of
five samples exceeding the general use criteria for manganese. Causes of low
dissolved oxygen typically include sediment oxygen demand, degradation of CBOD,
nitrification of ammonia and/or algal respiration. Potential sources contributing to
one or more of these causes include failing private sewage disposal systems (lesser
extent), runoff from agricultural land (fertilized cropland and agricultural land with
livestock), a permitted point source discharger, and an intensive animal feeding
operation. The observed manganese concentrations likely reflect natural background
conditions (soils in the watershed are naturally high in manganese), although brine
from oil wells may also contribute to the elevated concentrations. Because of the
naturally high levels of manganese in the soils, the general use criterion may be
difficult to attain.
• For Brush Creek (Segment CAR 01), the data, though extremely limited, are
considered sufficient to support the listing of this segment for dissolved oxygen and
