Chapter 10 Nutrient concentration levels in rural and suburban environments
Chapter summary:
- The level of nutrient concentrations in natural undisturbed streams reflects the general balance of respiration processes in aerobic environment
- In natural undisturbed environments, the level of inorganic nitrogen and phosphorus is very low as most of both atoms are stored in the organic pool
- The Haber-Bosch process and phosphate mining are at the root of the starkly higher level of N and P concentrations in disturbed rural streams
- The concentration level, the oxido-reduction level, and flowpath and source location of excess nutrient drives the type of treatment one might consider
- The chapter reviews traditional and newer indicators used to measure concentrations and loads in rural and suburban environments
10.1 Level of dissolved oxygen concentrations in open waters
The concentration of dioxygen in water determines the activity of electrons as dioxygen is a powerful electron acceptor, or ‘stealer’ as we have seen in this chapter. The concentration of any constituent in water is determined by the dynamic equilibrium between supply and demand rates. The concentration of dissolved oxygen (DO) does not escape this simple rule.
10.1.1 The supply and demand of O2 in water
The equilibrium between atmospheric O2 and that in the aqueous phase is discussed in details in this chapter. At this point, it is only important to understand that the activities, used in the chemistry sense here, are the same for elements in different phases at equilibrium. As a result, at equilibrium, there is O2 dissolved in water and the amount dissolved depends, in conditions where there is no demand, mainly on the temperature and on the velocity of water. The colder the water, the higher the concentrations, and the warmer the water, the smaller the concentrations. For example, at 5°C one can calculate that the equilibrium concentration is 12.8 mg/L, while at 20°C, it is 9.2 mg/L. We will see in this chapter that the reaeration of water, i.e., the kinetics at which O2 can dissolved into water depends on its turbulence: at low velocity, the reaeration is low, and at high velocity it is high. Additionally, the rate of supply of O2 to water may be affected by photosynthesis, which creates an endogenous additional source.
The demand of oxygen in water comes from all aerobic respiratory processes existing in the water column itself (e.g., mineralization, nitrification, etc.) plus those existing in the bottom substrate.
The final DO concentration results from the equilibrium between the supply and demand rates. When the potential for supply is much higher (e.g., swift cold stream) than the potential for demand, the DO concentrations are high (e.g., > 6 mg/L). To the contrary, when the potential for supply is low (e.g., stagnant warm wetland water) and the potential for demand high (e.g., the same warm organic rich wetland), the DO concentrations will equilibrate at low concentrations (e.g., < 2 mg/L). In the end, the level of concentration of DO, does determine the level of activity of aerobic respiratory processes in streams, which has consequences on the concentration levels of important inorganic and organic elements.