48 • Wetlands: Their Use and Regulation 



8 million gal/d (assuming 44 inches of precipita- 

 tion/yr) (56). 



The quality of the ground water resource also 

 determines the value of a particular recharge area. 

 WhUe Lawrence Swamp recharges large quantities 

 of water to the shallow aquifer direcdy underneath 

 it, this aquifer has a high content of fine sands, iron, 

 and manganese and cannot be used as a water sup- 

 ply (56). 



Water Quality Improvement 



By temporarily retaining pollutants, such as sus- 

 pended material, excess nutrients, toxic chemicals, 

 and disease-causing micro-organisms, it is generally 

 believed that wedands improve, to varying degrees, 

 the quality of the water* that flows over and 

 through them. Dissolved nutrients (i.e., nitrogen 

 and phosphorous) may be taken up directly by 

 plants during the growing season and by chemical 

 absorption and precipitation at the wedand soil sur- 

 face. Organic and inorganic suspended material 

 also tends to setde out and is trapped in the wedand. 

 Some pollutants associated with this trapped ma- 

 terial may be converted by biochemical processes 

 to less harmful forms; some may remain buried. 

 Others may be taken up by the plants growing in 

 the wedand and either recycled or transported from 

 it. 



The accumulation of toxic chemicals, such as 

 heavy metals and petroleum and chlorinated hydro- 

 carbons by wetlands may be only temporary (from 

 days to years). On the other hand, some toxic 

 chemicals have accumulated in many wedands over 

 a much longer time. With some toxic chemicals, 

 like degradable pesticides, the fact that these 

 pollutants are secured in the wetland long enough 

 to degrade is important. Other toxics either remain 

 buried or are taken up by the wetland plants. 



While wetlands may, under natural circum- 

 stances, retain nutrients on a net annual basis, the 

 value of a particular wetland for water quality im- 

 provement depends on the effect of the nutrient 

 storage on an adjacent or connected body of water. 

 However, even if a wetland does not retain large 



amounts of nutrients on a net annual basis, it may 

 influence the timing of nutrient inputs into adja- 

 cent waters. By retaining nutrients during the grow- 

 ing season, for instance, and exporting them after 

 the growing season, wetlands may have a positive 

 influence on water quality. Freshwater wetlands 

 have been used successfully for secondary treatment 

 of sewage effluents. 



Trapping Suspended Sediment 



Excessively high levels of suspended material in 

 the water column can be detrimental. By increas- 

 ing turbidity, suspended sediment can interfere with 

 fishing, swimming, and the esthetic appeal of water. 

 Reduction in light penetration due to increased tur- 

 bidity can kill aquatic plants, and settling of the 

 suspended sediment can smother bottom-dwelling 

 invertebrates and impair fish spawning. If sus- 

 pended sediment has a high organic content, the 

 dissolved oxygen level in the water column may de- 

 crease to levels that may adversely affect many or- 

 ganisms. 



One of the major water quality functions of wet- 

 lands is the removal of suspended sediment. By re- 

 ducing wave energy and the velocity of water flow- 

 ing through the wetland, wedand plants encourage 

 the deposition of suspended sediment. In fact, sedi- 

 mentation rates are related directly to the density 

 of marsh vegetation (7). Measurements of sediment 

 accretion, most of which are for marine or estuarine 

 environments, range from 0.04 centimeters (cm) 

 to 1,100 cm/yr.9 



The ability of vegetated wetlands to trap sus- 

 pended sediment more effectively than similar un- 

 vegetated areas was shown clearly in an 8-year 

 study on Currituck Sound in North Carolina. Dur- 

 ing the first 5 years, planted marsh lost an average 

 of 1 .4 m^/linear m of beach/yr, while an adjacent 

 unplanted area lost 3.3 m'/yr. Between 1978 and 

 1979 the planted areas, however, captured an av- 

 erage of 1 .5 m^ of sediment/yr; the unplanted area 

 lost an additional 1.3 m'. From 1979 to 1980, the 

 planted area gained 0.6 m' and the unplanted area 

 lost 0.4 m'. During the last year of the study, the 

 planted area appeared relatively stable, while the 

 unplanted area lost 1.0 m' (7). 



*The term "water quality" is defined here as the chemical, physical, 

 and biological condition of the water itself and not more broadly as 

 the condition of the wetland and its associated habitat. 



'Adamus and Stockwell, op. cit. 



