Report Number: 79
Year: 1997

Aqueous Chemistry of a Perennial Wetland in Southern Guam

Research is reported on the aqueous chemistry of soil pore waters and associated surface runoff from a small perennial riverine wetland astride the Taelayag River in southwestern Guam. The study took place over the onset and peak of the 1996 monsoon rainy season. The purpose of the project was to establish a first-order understanding of the chemical processes that occur within wetlands, especially sediment pore waters, so that ultimately we can establish how this, and presumably many other wetlands of similar origins in southern Guam, influence or buffer downstream water quality.

The Taelayag River winds through badlands terrain that has developed in soft volcanic rock. Badlands slope process and erosion contribute the sediment that builds and maintains the wetlands along the floodplain of the Taelayag River. Groundwater seepage from the wetlands and overbank flooding maintain the sediment balance and waterlogged conditions.

Chemical weathering and erosion in the badlands fractionates particle grain sizes and chemistry, and influences the chemistry of waters moving into the wetlands below. In the wetlands, redox- and pH-driven reactions coupled with bacterial activity alternately dissolve and precipitate metals. Filterable and total iron and manganese concentrations may reach three and two orders of magnitude, respectively, above ambient stream levels.

Low dissolved carbon, phosphate, and nitrogen levels are typical of Taelayag River wetland soil waters. The soils on local adjacent slopes that wash into the wetlands also are nutrient impoverished. It seems reasonable then to assume that the plant community is dependent for nourishment upon seasonal flooding and sedimentation from the main trunk stream.

Variation in chemistry across a given wetland can be considerable. Proximity to hillside runoff and depth in the soil column are significant sources of chemical variation. The major source of variation however is believed to be temporal, not spatial. Over the course of a given monsoon season, high ratios of water/soil promote elevated dissolved oxygen levels, increases in both Eh and pH, and dilution and short residence times of pore waters. Monsoonal flushing effects in these highly permeable wetlands override spatial variation that may develop over the rest of the year.

Downstream chemical effects on wetlands flushing were not investigated systematically. We noted, however, a pervasive buildup at the mouth of the Taelayag River of precipitated manganese and iron oxides on reef debris. Coatings on coral fragments may reach 1.5 cm thickness. Oxide coatings are not common nor as thick on reef detritus lying at the mouths of the adjacent Sagua or Taleyfac Rivers.

Galt H. Siegrist Jr.
Gary R. W. Denton
Rick H. Wood
Lucrina P. Concepcion
Russell R. Lewis