The Verde River provides approximately 40% of the surfacewater used by the Phoenix Metropolitan Area. However, upstream erosion and the risk of catastrophic wildfires could one day make water from the Verde River difficult to treat to drinking water standards. Turbidity, a measure of water clarity, was assessed against standards for water treatment in downstream communities.
How is it measured?
Although it would be ideal to measure turbidity throughout the watershed and use these measurements to grade each reporting region separately, sufficient data for this purpose did not exist at the time the Report Card was developed. Therefore, we graded turbidity in one location at the bottom of the watershed, where it enters municipal drinking water supplies in the Phoenix Metropolitan Area.
The City of Phoenix provided permission to use data from the Val Vista Water Treatment Plant (VVWTP) for the development of the turbidity indicator. Turbidity at the VVWTP events is heavily influenced by flows from the Verde River because other sources of water treated there (Central Arizona Project, Salt River, and groundwater) are all generally less turbid than water from the Verde.
Elevated turbidity events, called “Undesirable Events,” are instances where the average daily turbidity at the water treatment plant is greater than 500 Nephelometric Turbidity Units (NTUs, a measure of water clarity). This results in a condition where the water is more difficult to treat, but treatable. Water can become untreatable when turbidity excceds 8,000 NTU. In consultation with downstream stakeholders we assigned grades so that if current conditions are maintained the watershed would get an 'A.' Specifically, the watershed is assumed to score 100 in it's natural state and this score is reduced by 10 points for each Undesierable Event and 20 points for each Untreatable Event.