Topper Memo - Water Use by Individual Sewage S

• Basin Roundtable: South Platte
• Title: Water Use by Individual Sewage Systems
• Date: 5/7/2007
• Author: Ralf Topper
• Basin Roundtables - Doc Type: Needs Assessment Documents

STATE OF COLORADO

COLORADO GEOLOGICAL SURVEY —

serving the people of Colorado

Department of Natural Resources
1313 Sherman Street, Room 715
Denver, CO 80203
Phone: (303) 866 - 2611
Fax: (303) 866 - 2461

MEMORANDUM

To: South Platte River Basin Roundta ble
From: Ralf Topper, CGS liaison
Bill Ritter, Jr.
Governor
Date: May 7 , 2007

Harris D. Sherman
Executive Director

Subject: Consumptive/non - consumptive water use by residences using
Vincent Matthews
Division Director and
individual sewage disposal systems.
State Geologist

In February 1974, then State Engineer Jeris Danielson asked staff to investigate the consumpti ve
use of water by homes using leach fields for sewage disposal. In preparing a plan of
augmentation, developers relying on leach fields for effluent disposal were submitting the figure
of 10% consumptive use within the system. The State Engineer had ac cepted this value without
knowing whether or not the figure is accurate. Staff spent considerable time reviewing the
published literature, but found no direct studies pertaining to consumptive use of residential
septic systems. Literature with ancillary information useful to their investigation , however, was
obtained. In addition, a number of persons and agencies were contacted to solicit additional
information and input. Based on their findings, staff concluded that 80% of the water entering a
house wa s used by toilets and in bathing. Applying estimates for in - house consumption and
evaporation they determined that 8.4% of the water would be consumptively used before
entering the septic tank. Evaluating the results of a then new (1976/77) field investi gation by Dr.
Paul Trost in conjunction with consumptive use determinations using the Blaney - Criddle or
similar methods, staff determined that during the growing season approximately 9.6% of the
water was consumed within the leach field. On an annual basi s, this amounted to only 3.9%.
Thus on an annual basis the total consumptive use (in - house + leach field) was estimated at
12.3% (8.4% + 3.9%).

Even today, few Colorado site - specific studies related to individual sewage disposal systems
(ISDS) also refer red to as onsite wastewater systems have been conducted. Where those studies
exist, they predominantly focus on the water quality impacts to ground water. In February 2002,
Recommendations of the Individual Sewage Disposal System Steering Committee, esta blished
by the Executive Director of the Colorado Department of Public Health and Environment were
published. This committee identified issues with ground - water quality impacts, operation,
administration, and management of ISDS systems and made 13 recomme ndations; but none of
those dealt with consumptive or non - co nsumptive use. In response to h ealth department
officials, planners, and County Commissioners in Park County, the U.S. Geological Survey