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GROUND WATER MONITORING WORK PLAN DREDGE MATERIAL DISPOSAL SITE 1,ROBERTS ISLAND,PORT OF STOCKTON Water samples have not yet been obtained from geologic units typically associated with aquifer materials from below the peat layer. One of the objectives of this Work Plan is to characterize the shallow hydrogeologic system, including the shallow aquifer. 2.4 POTENTIAL SOURCES OF ACIDITY IN VALLEY FILL AND DREDGED MATERIALS The two most important acid-forming materials responsible for acidification are sulfides and ammonia. Based on a literature review of Mother Lode geology, specific minerals with the potential for acidity are present in the Valley Fill materials, and correspondingly, in the dredged sediments. The mineral iron pyrite is the dominant source of sulfides, and is readily observed in the dredged materials at DMD1. Oxidation of iron pyrite by bacteria produces sulfuric acid. Due to relatively high natural abundance, sulfides are likely to be the dominant source of acidity in Valley Fill materials and in the dredged sediments. Ammonia-bearing rock minerals, are also abundant in the Mother Lode geology. Samples of pre-dredged sediment revealed measurable concentrations of ammonia (USACE, 2003, 2004). Minerals in the Motherlode geology typically associated with ammonia include muscovite, biotite schists, greenstone, and slate. Oxidation of ammonia by bacteria can produce nitric acid. There are significant differences in the ratios of nitrogen isotopes derived from geologic materials versus atmospheric (meteorological) sources of nitrogen. Therefore, nitrogen isotopes may be a useful tracer when evaluating the potential migration of water in dredged sediments to underlying ground water. 2.5 PREVIOUS INVESTIGATIONS In addition to the data generated prior to proposed dredging activities, several investigations have been performed at DMD1. • In March 2001, four ground water monitor wells (Figure 2) were installed. Ground water samples have been collected in March 2001, September 2002, May 2003, and May, September, and December 2004. The ground water samples were analyzed for a variety of metals,metalloids, general minerals, and pesticides. • In October 2002, sediment pH monitoring began at depths of 6 inches and 18 inches bgs at three locations within Area A. Sediment samples have been collected and analyzed for pH on 25 separate events through December 2004. • In October 2002, five shallow borings were advanced in Area A to approximately 4.5 feet below ground surface (bgs). January 2005 Page 6 Environmental Risk Services,Corp.