Dissolved Oxygen Modeling in Surface Waters

An ORP electrode is inserted to a point 0.3 cm below the sediment-water interface of a stratified lake in late August. A voltage of approximately 0.25 V is recorded. Assuming for the moment that ORP is a crude estimate of Eh, what reactions are most likely to be occurring in the sediment at this depth  

As shown in Fig. 2-21, iron oxides and manganese oxides are most likely the chemical species being used as oxidants in the degradation of organic matter at this pe. 

Upstream of a sewage outfall, a river contains 7 mg/liter DO. Some distance downstream of the outfall, however, DO has been diminished to 4 mg/liter due to organic waste decomposition by microbes. What is the approximate amount of organic matter ( CH20 ) that must have been degraded to account for this consumption of DO  

Neglecting 02 diffusion into the water from the atmosphere and 02 production by photosynthesis, and assuming no 02 consumption by organisms other than the microbes, 3 mg/liter 02 are consumed. This corresponds to 

The degradation of 2.8 mg/liter of organic matter thus consumes 3 mg/liter of dissolved oxygen. Actually, some 02 from the atmosphere will have dissolved into the stream, so 3 mg/liter is a minimum value for 02 consumption. 

---

It follows that deep seawater contains nutrients from two sources. First, it may contain nutrients that were present with the water when it sank from the surface. These are called "preformed nutrients". Second, it may contain nutrients derived by the in situ remineralization of organic particles. These are called oxidative nutrients. The oxidative nutrients can be estimated from the RKR equation. From this model, we might expect the four dissolved chemical species (O2, CO2, NO3, PO4) to vary in seawater according to the proportions predicted. The key to understanding these remineralization reactions is the parameter Apparent Oxygen Utilization (AOU), defined as ... 

---