The effects of unpaved access roads on runoff and associated water quality within the Seele Estate, New Hanover, South Africa.

Abstract

Unpaved forestry roads can significantly affect surface runoff and sediment production, with consequential impacts for stream water quality. The potential impact of road runoff on stream water quality is mitigated by the redistribution of runoff into the forest compartments through road drains. The objective of this study was to assess runoff and the associated nutrient loads from unpaved forest access roads, and to evaluate the effectiveness of road runoff redistribution onto the forest compartments. Unpaved road segments in Mondi Forest Plantation in Seele Estate, New Hanover, South Africa were instrumented for runoff measurement in response to natural rainfall. Two road segment classes were investigated for water quality from unbounded runoff plots: steep sloped road segments of road gradients of 9.5° and 7.5°, and gentle sloped road segments of road gradients of 1.6° and 2.0°. Water quality was also assessed by monitoring road runoff, and stream water quality was analysed for water quality parameters including; pH, Nitrates, Nitrites, Phosphates, Total dissolved Oxygen, Oxygen consumption, Ammonium and temperature upstream and downstream of the Estate. The effectiveness of road runoff redistribution into the forest compartments was evaluated through relating water distribution to tree breast height diameter. Two sets of road drains corresponding to the plots of different road gradients were selected as for runoff, and sampled, and corresponding plots or allotments were established to determine tree breast height diameter measurements. The results of the study revealed that, as might have been expected, runoff production increases with the increasing road gradient. The quality of road runoff water was lower than the stream water. There were no significant differences observed in nutrient levels upstream and downstream of the road stream crossings. The nutrient concentrations however, were higher upstream of the estate than downstream. Significant differences in tree breast height diameter were noted between plots of different road gradients. This suggested that the gradient determines the infiltration of redistributed runoff and hence the availability of the water that can be used by the trees within a compartment. The results of the study suggest that unpaved roads are important in the generation of nutrient loads. Much of the nutrient value is redistributed within the compartment itself rather than being transferred to the stream. This suggests that, provided that road runoff can be contained within the compartments, the potentially negative impact of road runoff can be mitigated and may enhance tree growth.