IMPROVING TASTE, ODOR AND APPEARANCE

If taste, odor or appearance problems still occur after one or more of the treatment methods described above, other solutions are available that treat the problem of taste or odor at the source. If the process itself is a source, such as chlorine odor, the problem can be solved through modifying the process or aerating the water. If the source is an event like an occasional algae bloom, the use of activated charcoal may solve the problem. Researchers at Arizona State University are currently researching ways to control taste and odor problems.

Blending

One way to mitigate some water quality problems is to blend water containing unacceptable levels of some contaminant with water that has little or none of the same contaminant. For example, hard water can be blended with softer water to produce water somewhere in between. Water that contains an unhealthy level of a toxic such as TCE can be blended to lower the TCE level below the danger point. Blending, however, is not always as simple as it may appear. Most studies have shown that blending would improve CAP water quality. Tucson Water’s latest proposal is for a blend of approximately 45 percent CAP water with 55 percent groundwater.

Soil (Alluvium)-Aquifer Treatment

Some utilities percolate wastewater or river water through the soil for at least part of the water treatment process. The water is then pumped back up for use. Several pilot projects in the Tucson area have been conducted, and research is continuing into the ability of soil and underlying materials to remove contaminants. (The use of the term “soil” is not entirely appropriate because the soil often is scraped off to construct the basins, but since soil is the common term, it is used here.)

Experiments show that soil-aquifer treatment (SAT) removes almost all pathogens, more than 90 percent of organic matter, more than 80 percent of total halogen, and none of the volatile organic compounds such as TCE or dissolved minerals or salts. In other words, SAT can be used for disinfection but must subsequently be treated with chlorine or another disinfectant to ensure safety and to retain residual disinfection in the distribution system. The size and to some degree the composition of the alluvium particles determines such factors as rate of percolation, clogging of the pores by algae and bacterial growth. Clogging can be reduced by alternating wet and dry cycles. This provides time for any algae to die before the water is again applied. Some soils are more porous than others, and the rate of percolation may change over time as pores fill with water. Some pollutants, such as those that are found at landfills, may appear in the water. While some utilities use SAT to treat waste water, no major utilities use the method to treat water for drinking purposes.

QUALITY OF CAP WATER

CAP water is generally of high quality. It contains no problem levels of toxic materials, and the low levels of microbes are easily treated. The water comes from the Lake Havasu section of the Colorado River. Colorado River water of similar quality is served to more than 25,000,000 people in southern California, Nevada and Arizona. In Arizona and Southern California the water may be blended with water from other sources or used without blending. Some examples of how other communities use Colorado River water are given below.

An important difference between CAP water and Tucson groundwater is the corrosivity of the water. CAP water is more corrosive than Tucson groundwater partially because it contains higher levels of sulfate and chloride. From their experience with CAP water some Tucsonans fully realize that corrosive water can seriously damage pipes and fixtures. As described above, corrosivity depends on pH, TDS, temperature and dissolved oxygen as well as the composition of the pipes.

Another major difference is the level of salinitY. (This refers to dissolved minerals, generally not table salt.) Salinity of the groundwater varies greatly according to location, with average salinity in the Avra Valley about 210 mg/l and levels up to 2,500 mg/l detected in groundwater near Green Valley and the mines. As is shown in Figure 6-6, the salinity of water delivered to Tucson Water customers also varies widely. Different sets of wells serve different parts of the distribution system, causing salinity of water delivered to the customer to range from below 200 mg/l to above 650 mg/l. In general, as pumping continues and water levels decline, the salinity of pumped water is expected to increase.

The salinity level of CAP water varies seasonally and from year to year depending on flow conditions on the Colorado River. Higher than average flows generally dilute salinity. In 1995, the average annual salinity of Colorado River water below Parker Dam, when adjusted to average flow conditions, was 775 mg/l – or more than double the average of Tucson groundwater. The TDS of CAP water delivered to Tucson varies also, depending on a number of factors. (Total dissolved solids is approximately the same as salinity.)

As is indicated in Figure 6-7, Colorado River salinity comes from both natural processes and human activities. Natural processes account for much of the salinity, as the river and its tributaries pick up minerals from certain kinds of rocks and soils. Highly saline springs also raise the salinity levels. Irrigation contributes salinity by consumptively using water, concentrating the salinity of the water left behind and by returning water to the river with additional dissolved minerals from soils high in mineral content. Reservoir and canal evaporation, including the evaporation that occurs as water is delivered via the open CAP canal, increases salinity concentrations because as water evaporates, less water is left to dilute the same amount of salts.

The salinity of the Colorado River starts at about 50 mg/l in its mountain headwaters and increases to over 800 mg/l at the Mexican border. Salinity levels have been steadily increasing throughout the lower basin and Mexico as more river water is used and water evaporates from the reservoirs. As a result salinity levels have become a serious concern. Federal and state programs have been enacted to remove salt loading sources on the river and prevent further increases in TDS. These programs have removed thus far approximately 140,000 tons of annual salt load on the river. Continued Congressional funding of these programs, however, is in question. If salinity control programs are cut back or eliminated, TDS levels in the future are projected to exceed 900 mg/l at the CAP diversion point on the river.