Report Number: 3
Year: 1977

Agat Sewage Treatment Plant: Impact of Secondary Treated Effluent on Guam Coastal Waters

The purpose of this study was to obtain long-term information on the impact of a secondarily treated sewage effluent on Guam' coastal waters. Water samples from the effluent treatment tanks and receiving marine waters were analyzed for coliform, NO3-N and reactive PO4. Marine biota, especially benthic algae and zooplankton, were quantified in the vicinity of the outfall.

The results of the study of effluent in the treatment plant reveal that nutrient levels are high, with reactive PO4 varying between 170 and 400 μg-at/L (mean = 260 μg-at/L) and NO3-N ranging from 10 to 530 μg-at/L (mean = 130 μg-at/L) Forty-six coliform samples from the effluent were less than 200 counts per 100 mL. Fecal and total coliform counts taken from the raw sewage prior to chlorination yielded counts in excess of 105 per 100 mL.

The effluent discharged from the outfall pipe rises immediately to the surface. Oxygen levels in surrounding waters reach saturation within 1 m of the discharge point. Freshwater diffusion occurs rapidly in the surrounding waters. Surface phosphate levels are reduced to 1/3 of that originating at the point of discharge. Upon reaching the surface, the fate of the effluent depends on wind, waves and current conditions existing at any given time. Under heavy surf conditions, the effluent is washed toward the seaward edge of the jetty and then deflected north around the jetty. The effluent is carried by prevailing winds when winds are of sufficient strength (ca. 15 knots) and surf is minimal. In the absence of significant swells and wind velocity, the effluent moves on the surface in sluggish eddies, eventually drifting offshore.

The additional nutrient input from the sewage to the marine environment has little effect on benthic biota since the nutrients are carried immediately to the surface. Phosphate levels just above the substratum, adjacent to the point of discharge, are low and similar to the control samples offshore (mean = 0.40 μg-at/L). The change in the amount of benthic algae, especially Galaxaura oblongata, in this area probably reflects a seasonal change as well as relation to antecedent events. Live corals are few, comprising less than 3% coverage. No change in coral coverage was evident. The fish in the area varied in number and diversity depending on the condition of the sea. Zooplankton (per unit volume displaced) are consistently higher in the control sample obtained from an area south of the outfall just off the reef platform. Copepods and fish eggs dominate both outfall and control samples obtained during the day; shrimp larvae dominate outfall samples taken at night.

Roy T. Tsuda
Deborah A. Grosenbaugh