In general, rainwater runoff from the overhead portions of structures represents a diffuse source of contaminants whose assessment is not amenable to the proximity model approach for predicting environmental concentrations of contaminants associated with immersed treated wood. During the last 20 years, rainwater runoff studies have allowed an assessment of the contribution of contaminants from overhead structures. With this new information, this model includes elements of a box model for assessing timber bridges, and this approach is recommended for all structures having large overhead (out of the water) leaching surface areas and/or numerous immersed treated wood members.
The box model employed is based on the following assumptions:
- Current speeds can be characterized using a mean current speed collected along a vector orthogonal to the shoreline at the average water depth where treated wood is placed. This assumption greatly simplifies the model because both the preservative losses and the volume of the receiving water diluting the preservative are functions of water depth. Therefore, the model need only consider the amount of preservative lost to a unit depth of water and the volume of water in that unit depth that passes by the structure with some measurable speed.
- Pressure treated wood of known dimensions contributes contaminants to a box that is defined by the width of the structure orthogonal to the currents; the current speed; and the water’s depth.
- The amount of preservative delivered into the box is determined by multiplying the loss rates by the total leaching surface area of immersed treated wood and the predicted contaminant levels in rainwater runoff from the overhead structure by the volume of rainwater. Sediment concentrations accumulate over an extended period of time and the model assumes that rain falls at a steady rate for the life of the project (generally 35 years). The model includes provisions for estimating increased contaminant concentrations associated with short term storm events that do not affect the flow of water under the structure.
- The volume of dilution water is the width of the project measured orthogonal to the currents*the current speed*the period of time used in the loss algorithm*the water’s depth. For environments that are dominated by tidal currents, the worst case (minimum dilution) is assessed during the period within half an hour either side of slack tide.