COM 0865.001 1996-1998 - Laserfiche WebLink

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Draft Environmental Assessment June, 1998 Old Hilo Wastewater Treatment , .it Page 9 6.6 Water Bodies -r- Groundwater beneath the project site is identified as the Hilo aquifer system within the Northeast Mauna Loa aquifer sector (Mink, 1993). The aquifer system is a voluminous basal lens that extends at least four miles inland from the shoreline. The aquifer is noted for having an enormous groundwater flux. The flux from fresh water springs has been measured at 150 mgd. The hydraulic gradient has been measured at 5 feet per mile, or 1 per mil. The Hilo aquifer system is classified as an irreplaceable source of fresh drinking water. The aquifer system is currently used as a drinking water source, and the vulnerability to contamination is high (Mink, 1993). The groundwater flow direction in the area is towards the ocean. The fresh groundwater discharge along the coast is many times larger than surface flow. The subject property lies on the shoreline of Puhi Bay. The area has vast ocean water resources. ¦ The average temperature of surface seawater 75 °F. The seawater at a hundred feet below the ground is 68 °F, and at 3400 feet deep is about 40 °F. . 6.7 Natural Hazards The Old Hilo WWTP facility is susceptible to various types of hazards including coastal flooding, tsunami inundation, volcanic activity and earthquakes. The site lies in a special flood hazard area inundated by floods from 100 year storm events. The site is designated by FEMA to be Zone VE (Figure 5). This means that the property is subject to coastal flooding combined with wave action. Base flood elevations range from 18 to 20 feet on the property. Hilo, with its exposure to a crescent shaped bay and orientation towards the Pacific seismic belt is very susceptible to tsunamis. Forty-three destructive tsunamis have reached Hilo since 1819. Frequency analysis by the U.S. Army Corps of Engineers led to development of a frequency analysis curve and tsunami inundation zones. Figure 5 reveals that while inundation of 25 feet at the shoreline may only happen once or twice every hundred years, waves of two feet may occur 20 times in a hundred year time span. The abandoned treatment facility lies on the shoreline within the inundation zone. The most common volcanic hazard in Hawaii comes from lava flows. Generally, there is very little direct risk to human life, but risk to property can be great. The greatest danger from volcanic activity for the Hilo area is from the northeast rift zone of Mauna Loa. Since 1880, most lava flows from Mauna Loa stopped prior to reaching the urban areas of Hilo. However, the danger of future lava flows reaching and destroying any part of Hilo is present at all times. The entire island of Hawaii lies in seismic zone 3. The risk of damage from earthquakes is considerable for all areas of the island. Earthquakes may be expected in the Hilo area in the future. oi'