Water quality modelling to the rescue!

The anthropogenic activities as discussed in the previous post such as agricultural, municipal, industrial activities and contamination from those activities introduce large amount of nutrients and organic materials that contribute to eutrophication and depletion of oxygen. It goes without saying that the impacts of low DO levels or in extreme conditions, anaerobic conditions can cause fish mortality, odours and unbalanced ecosystem.Hence, a water quality policy should make sure that the pollution's concentration are kept below the acceptable standards and that DO levels meet the minimum concentration for the aquatic animals (Prakash et al., 2007)

A study was carried out by Prakash et al. (2007) for river Bagmati in Nepal. QUAL2Kw model was used for water quality and dissolved oxygen control.

In order to achieve the standard target of minimum DO concentration of 4 mg/L, the assimilative capacity of the river should be sufficient. And this can be achieved by first controlling the river flow rates, secondly by controlling the pollution loads, and finally by applying oxygenators (Campolo et al., 2002).

The study area was about 20Km length of the river, which has religiously and culturally significant to Hindu-Buddhist society. The river was highly appreciated for its purity decades ago. But that is changed due to the pollution. 

Women taking a holy dip in the river- source

The study divided the river into 41 reaches with length of 0.5 km each. There was 11 monitoring stations that monitored different parameters such as pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), total nitrogen (TN), total phosphorus (TP), river flow (Q), river velocity (m) and river water depth (m), etc.

The results from the model are shown as continuous lines and the observed data as symbols. The results were in well agreement with the observed data with some exception. The calibration result showed that the water quality is below the minimum standard for DO beyond 8km.

Figure 1: Calibrated results for Bagmati River- source

The model was then used to simulate different water quality management strategies such as pollution load modification and local oxygenators to keep the DO levels within acceptable threshold. The combination of all three proposed methods of enhancing the DO levels is necessary to ensure the target is achieved. 

The figures below show the simulation results first with only changing the CBOD concentration (indication of pollution), then combining the modification of CBOD and flow augmentation, and finally the combination of all three of the proposed method.

Figure 2: DO profiles along  the river for different CBOD and 0.3 mg/L TN limits at point loads- source

Figure 3: DO profiles along Bagmati River for different CBOD and 0.3 mg/L TN limits at point loads
with 1 m3/s flow augmentation- source

Figure 4: DO profiles along the river for different CBOD, 0.3 mg/L TN limits at point loads with 1 m3/s
flow augmentation and weirs - source

Hope that this showed how modelling is used both in assessing the water quality as well as in coming up with options for management.