Figure 1. A baldcypress and tupelo gum forest receives wastewater 

effluent from the aerated holding pond shown beside the Breaux Bridge 

Wastewater Treatment Plant.

U.S. Department of the Interior

U.S. Geological Survey Printed on recycled paper Fact Sheet 104-02September 2002

Effects of Wastewater on Forested Wetlands

Cycling nutrient-enriched wastewater from holding ponds 

through natural, forested wetlands is a practice that municipal 

waste treatment managers are considering as a viable option for 

disposing of wastewater. In this wastewater cycling process, 

sewer effluent that has been circulated through aerated ponds is 

discharged into neighboring wetland systems. To understand 

how wastewater cycling affects forest and species productivity, 

researchers at the USGS National Wetlands Research Center 

conducted dendroecological investigations in a swamp system 

and in a bog system that have been exposed to wastewater 

effluent for many decades. 

Dendroecology involves the study of forest changes over time 

as interpreted from tree rings. Tree-ring chronologies describe 

the pattern and history of growth suppression and release 

that can be associated with aging and disturbances such as 

hurricanes, floods, and fires. But because of limited monitoring, 

little is known about the potential for long-term effects on 

forested wetlands as a result of wastewater flooding. USGS 

researchers used tree rings to detect the effect of wastewater 

cycling on tree growth. Scientists expected to find that tree-ring 

width would be increased as a result of added nutrients. 

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.

Baldcypress in the treated site showed a pattern of enhanced 

growth that corresponded to the onset and duration of wastewater 

cycling. The treated group also exhibited greater growth 

following wastewater application unlike the control group, which 

exhibited greater growth prior to wastewater cycling. Despite 

the greater hydroperiod for the treated site, the addition of 

nutrient-enriched wastewater augmented tree growth for over 40 

years and improved the health and productivity of the forested 

swamp ecosystem. These findings indicated that baldcypress is 

responsive to nutrient-rich wastewater and can sustain long-term 

exposure to effluent flooding.

Drummond Bog, Wisconsin

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. 
Figure 2. Thomas Doyle, a USGS scientist, is pictured in a Wisconsin 

bog coring a black spruce for tree-ring analysis.

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.

Conclusions

Growth chronologies of species from the Cypiere Perdue 

Swamp and Drummond Bog sites showed that nutrients from 

wastewater application over several decades can enhance forest 

productivity in both swamp and bog systems. In cases where 

wastewater stays impounded, such as in controlled ponds, bogs, 

or other depressional wetlands, less flood-tolerant tree species 

may be negatively impacted by prolonged flooding. Results 

indicate that natural, forested wetlands respond favorably to 

long-term wastewater cycling and thus, provide municipalities 

with a cost-effective alternative for disposing wastewater in a 

beneficial manner for humans and nature. 

Reference

Hesse, I.D., Doyle, T.W., and Day, J.W., 1998, Long-term 

growth enhancement of baldcypress (Taxodium distichum) 

from municipal wastewater application: Environmental 

Management, v. 22, n. 1, p. 119-127.

For more information, contact

T.W. Doyle

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

337-266-8500

thomas_doyle@usgs.gov

http://www.nwrc.usgs.gov