Effects of Wastewater on Forested Wetlands

	Figure 1. A baldcypress and tupelo gum forest receives wastewater effluent from the aerated holding pond shown beside the Breaux Bridge Wastewater Treatment Plant.

Cypiere Perdue Swamp, Louisiana

Forested swamps of the Mississippi River deltaic plain have been cut off from the natural overflow of the Mississippi River and its distributaries. As a result, these systems have been starved of episodic sediment and nutrient replenishment from overbank flooding. Since 1953, a municipal treatment facility in Breaux Bridge, Louisiana, has been discharging wastewater effluent into the Cypiere Perdue Swamp, a baldcypress (Taxodium distichum) and tupelo gum (also known as water tupelo; Nyssa aquatica) forest community in the upper reaches of the Vermilion River watershed (fig. 1). Such introduction of wastewater effluent into the forested systems of Louisiana may have the added benefit of increasing forest productivity and slowing the regression of coastal swamp forests into marsh.

To determine the effects of wastewater cycling in the Cypiere Perdue Swamp, scientists from USGS and Louisiana State University selected and cored mature baldcypress trees of similar size and age from both a treated site and a control site in the same drainage system above and below sewage treatment plant outfall, or discharge location. Individual trees were tagged and measured, and duplicate core samples were taken for ring definition and dating. With the aid of a microscope, ring growth was measured to an accuracy of 0.01 mm (0.0004 inch) for the years 1920-92. This sampling period includes over 30 years of growth that predate wastewater cycling, as well as 30 years that show its effects. Growth chronologies were developed for each tree set from the treated and control sites. For analytical purposes, these chronologies were composited by tree and site into equal growth periods over the pretreatment interval of 1926-52 and the treatment interval of 1953-88.

Figure 2. Thomas Doyle, a USGS scientist, is pictured in a Wisconsin bog coring a black spruce for tree-ring analysis.

A municipal treatment facility near Drummond, Wisconsin, has been circulating wastewater effluent through a forested bog since 1979. In preparation for effluent discharge and retention, a weir was constructed to impound surface waters in the 9-ha (22-acre) peat bog. The forest vegetation is a fairly even mix of black spruce (Picea mariana) and tamarack (Larix laricina). USGS scientists selected dominant, mature trees of both species from a treated site and from a control site for coring to determine the effects of cycling wastewater in Drummond Bog (fig. 2). The treated site was located downslope of the flow of wastewater within 15 m (49 ft) of the wastewater discharge pipe. The control site, which is upslope more than 75 m (246 ft) from the discharge pipe, was distanced from any apparent influence of effluent flooding.

Researchers used tree-ring analysis to document the effects of wastewater effluent on forest productivity. Individual trees and sets of pairs were measured for diameter at breast height, crown ratio, crown class, hummock area, and distance from discharge pipe. Duplicate core samples were taken, processed, and polished for ring definition and dating. Ring growth was measured for the entire lifespan to an accuracy of 0.01 mm (0.0004 inch) with the aid of a microscope. Growth chronologies were calculated by year and species for the treated and control sites. For analytical purposes, the chronologies were further composited by tree and site into equal growth periods over the pretreatment interval of 1964-78 and treatment interval of 1979-93.

Tree-ring data chronicled the establishment, disturbance history, and growth dynamics of a cohort, or age class grouping, of spruce and tamarack dating to the turn of the 20th century. Tree sample sets were of similar age and size. Findings showed that treated tamarack responded favorably to increased nutrient levels in contrast to the control group, while black spruce was negatively affected by a raised water table and prolonged flooding on both treated and control sites.