Effects of drainage of a forested peatland on water quality and quantity

Forest drainage is recognized as a means of improving soil conditions for tree growth, but it may also have environmental impacts. This article presents the 5-y time trend of soil and surface water quality observed in a drainage experiment conducted in a black spruce ( Picea mariana (Mill.) BSP) peatland of eastern Québec, Canada, using the paired watershed approach. Nutrient content of peat soil water was enhanced by drainage. The effect was generally proportional to ditch closeness for S and Mg, while increases of N, Na, K and Ca were mainly observed within 5 m of the ditches, even though high levels of K and Ca were occasionally observed at 15-m distance. Drainage increased and sustained summer low flows by 25% through a 80 m 3 d −1 increase of base flow on the 8 ha basin. The concentration of suspended sediments significantly increased and exceeded acceptable limits for aquatic organisms only during ditching and during the peak flows of the following weeks. No effect on suspended sediments was detected during the spring following ditching but an increase was observed during a storm 2 y after ditching. Drainage modified the basin outflow temperature fluctuations by lowering the average weekly minimum and increasing the maximum, by 2°C and 7°C, respectively. Water temperature increases to 25°C or more at the outlet of the drained basin were above the preferred range for brook trout. The treatment significantly increased specific conductivity of surface water, which was clearly related to increases in mineral N (NH 4+NO x ), Ca, Mg, Na and S concentrations. However, conductivity and the increased nutrient concentrations remained within the acceptable criteria for aquatic organisms. Drainage also produced a pH increase of one unit, a result attributed to increased runoff from the upland part of the treated watershed. No effect related to drainage was found for Zn, Fe and Al contents.