Potential Impacts of aSeawater Intake Structure

and a Brine Disposal System for the Proposed

Desalination Plant at Mile 6, Swakopmund

Marine Ecological Aspects

Environmental Services (Pty) LtdPISCES

Nina Steffani & Andrea Pulfrich

Intake Structures:Alternative 1: Direct open water intake structure with caisson at 8 or 10 m depth (intake mid-depth) 800 m to 1500 m offshore. Connected to onshore pump station through a pipeline that is trenched through the surf zone and beyond the surf zone placed on the sea bottom. On the shore, the pipeline will run underground into the pump station.

Alternative 2: Breakwaters extending 147 m out to sea to create a stilling basin of ~2 m water depth with an open channel connecting the stilling basin with the intake pump station.

Discharge Structure:~ 60% of the seawater will be returned to the sea as brine via a 600 m long pipeline. The brine will have approximately double the salinity of seawater.

Key Sources for Potential Impacts on the Marine Environment:

Construction of intake and brine disposal structures

Permanent presence of these structures

Discharge of high-salinity brine

Marine Ecological Aspects

Main Environmental Issues (Intake and Discharge Structures):

Construction Phase: High vehicular traffic on beach Accidental spillage or leakage of fuel, chemicals, or lubricants Increased turbidity and resettling of sediments during excavation Destruction of biota in the excavation area (inter- and subtidal) Disturbance to marine life from blasting Disturbance to seabirds

Operational Phase: Permanent presence of intake and discharge structures results in loss of original substratum but provides new settlement ground (e.g. artificial reef) Changes in hydrographical conditions and sediment transport; changes in wave patterns may affect beach morphology; due to sedimentation dredging may be required in future (breakwater) Impingement and entrainment of marine organisms Avoidance behaviour of fish, marine mammals, loss of potential feeding or breeding ground

Marine Ecological Aspects

Elevated salinities in the mixing zone and potential seafloor spreading Discharged effluent potentially having a higher temperature than the receiving environment Residual chlorine and chlorination by-products in the effluent (if no dechlorination step) Co-discharge of constituents with the brine (e.g. increased turbidity from backwash water and discoloration near outlet from FeCl3, heavy metals from corrosion processes, antiscalants may bind nutrients and ions dissolved in seawater, cleaning solutions for membrane maintenance may interfere with natural processes of dissolved seawater constituents) Removal of particulate matter with the feedwater from the water column where it is a significant food source, as well as changes in phytoplankton production due to changes in nutrients, water column structure and mixing processes (can cause algal blooms) Direct changes in dissolved oxygen content due to the difference between the ambient dissolved oxygen concentrations and those in the discharged effluent (e.g. due to dechlorination with sodium bisulfite), and indirect changes in dissolved oxygen content of the water column and sediments due to changes in phytoplankton production as a result of nutrient input

Marine Ecological Aspects

Main Environmental Issues (Brine Discharge):

Approach

Desktop study based on existing literature Field-work to establish type of community around brine outlet (thin sediment cover over bedrock, visual inspection by diving required) International guidelines provide target values (e.g. ANZECC, DWAF, World Bank) for e.g. salinity, residual chlorine Based on target values and guidelines assess impact of brine plume footprint determined by modelling (CSIR)

Marine Ecological Aspects

Information Requirements:

Exact design of both intake structure alternatives: type of seawater intake (screen type, mesh size, intake volume, intake velocity, etc.) Construction activities (temporary jetty, blasting schedule, etc.) Exact composition of brine (concentration of residual chlorine, concentrations of co-discharged constituents, discharge volumes, outfall design)

Marine Ecological Aspects