5 Colo. Code Regs. § 1002-32.14
The provisions of 25-8-202(1)(b) and (2); and 25-8-204; C.R.S. provide the specific statutory authority for adoption of the attached regulatory amendments. The Commission also adopted in compliance with 24-4-103(4), C.R.S., the following Statement of Basis and Purpose.
BASIS AND PURPOSE:
The purpose of the water quality standard for TDS adopted for Horse Creek is to help protect downstream agricultural and domestic water supply uses, to the degree feasible, taking ambient water quality conditions into account. This standard is intended to operate in conjunction with the Cheraw Lake control regulation, which is being adopted concurrently. The purpose of the control regulation is to protect the agricultural uses of water in Horse Creek (Otero and Bent Counties) from the highly saline discharges from tributary Cheraw Lake, while also avoiding an unacceptable adverse impact on other downstream water uses, particularly domestic water supplies.
The regulation takes into account the intermittent nature of any discharges from Cheraw Lake, and the possibility that any discharge could be diluted before the water reaches Horse Creek. The regulation does not impose any specific treatment or best management practice requirements. Rather, it provides flexibility regarding the means of compliance, so long as the specific level of salinity can be achieved in Horse Creek.
The saline condition of water in Cheraw Lake appears to be caused by highly alkaline native soils in the area together with routing of irrigation return flows to the lake. Traditionally, topography and the shortage of water in the Arkansas River Basin has prevented the lake from overflowing into Horse Creek. Evaporation losses then contributed to the increase in salinity which has exceeded 17000 mg/l (TDS) in the upper layer and 60000 mg/l at the bottom of the lake based on samples collected by the Division and the USGS. The excess of water caused by the past "wet" years has caused levels in the lake to rise significantly which, in turn, threatened to cause property damage to State Highway 109 and the Town of Cheraw. This led several parties to effect releases from the lake which have damaged and endangered the agricultural use downstream on Horse Creek. This statement is supported by the EPA "Red Book"criteria for irrigation water and Division and USGS water quality investigations of the Lake and Horse Creek.
The ambient quality of Horse Creek has exceeded 5000 ppm TDS without influence from Cheraw Lake based on the existing water quality database. Since the agricultural use of Horse Creek water was not impaired under those conditions, salinity levels of Horse Creek will be controlled based on the 85th percentile of 65 USGS measurements of specific conductance prior to the 1985 releases from Cheraw Lake. This value was then converted to TDS using a linear regression developed by USGS and Division staff. The calculated TDS standards is 4300 mg/l.
PARTIES TO NOVEMBER, 1988 HEARING
5 CCR 1002-32.14