David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Types of Water Pollution•We can classify types of pollution in a few different ways:• Source– We can distinguish between point sourcepollution, where the origin of the material can be identified and measures can be taken, and non-point source pollution, where there is no singular source of the pollution, usually because the material originates throughout the landscape• Ecological Impacts– We can group pollutants based on how they effect the ecosystem, e.g. pollutants that lower dissolved oxygen content vs. pollutants that mimic estrogenic compounds etc.David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Types of Water Pollution• General Types – We can group pollutants based on the nature of the material (or energy) in question:1. Heat (thermal) pollution – As we heat the water (either by returning heated water to the system directly, or indirectly), the solubility of dissolved oxygen in the water is decreased2. Silt & Sediment – Erosion of soils from a variety of activities increases turbidity, which decreases light penetration, decreasing photosynthesis in aquatic plants, which lowers the dissolved oxygen content3. Nutrients – Through careless fertilizer use, increased N and P can boost algal growth, again blocking light penetration etc.David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Types of Water Pollution4. Organic Wastes – When we add organic material to wastewater effluent, it can decompose in the water and reduce dissolved oxygen content5. Microorganisms – Also associated with organic material, these can transmit infectious disease6. Acids – Whether through acid precipitation or industrial effluent (including mining), by changing the pH of surface waters we can create conditions where some key organisms in an ecosystem cannot survive7. Various chemicals – Any number of toxic compounds, such as heavy metals, organochloranes etc., sources from pesticides and industrial discharges can be damaging to organismsDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Dissolved Oxygen• The concentration of dissolved oxygen (DO) in fresh water is a particularly important criterion of water quality, because most aquatic life depends on a certain amount of oxygen being present•A few processes can reduce the DO content of water:1. Dumping organic waste in the water provides organic carbon compounds in the presence of DO and various decomposers, thus organic waste + O2Æ CO2+ H20 + various compounds2. Aquatic plants photosynthesize and provide the DO, but excessive algal growth due to nutrients slows this3. Some aquatic organisms use the DO (e.g. fish etc.)David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Henry’s Law• In order to model the dissolved oxygen content in an aquatic ecosystem, we first need to understand the physical law that determines how much dissolved oxygen the water can hold• The amount of DO that water can hold is a function of Henry’s constant and partial pressures, based on Henry’s Law:CO2= KO2* PO2where: CO2is the concentration of DO in H2O (mg/L)PO2is the partial pressure of O2at the atmosphere – water boundary (atm)KO2is Henry’s Constant which varies inversely with temperature (mg/L*atm)David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Biochemical Oxygen Demand• From our previous listing of processes that diminish the DO in an aquatic system, we know that adding organic waste can reduce DO levels through the decomposition of that waste• The amount of oxygen required to decompose a certain amount of waste is its biochemical oxygen demand (BOD), and like DO is measured in concentration units (such as mg/L)• BOD is a useful way to describe the general water quality of a sample because it indirectly measures the amount of organic waste present in the water• Ultimate BOD (BODult) is used to describe the total amount of DO required to oxidize all the organic wasteDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Processes and Model Structure• We will model the resulting DO concentration in the river using a two-step approach:1. We will establish initial values of DO and BOD immediately after the river water and effluent are mixed using a mass-balance approach2. We will predict the expected downstream DO levels, taking into account some processes that will occur as the water moves downstream that change the DO:• Reoxygenation (or aeration) will absorb O2from the atmosphere as the water moves downstream• Deoxygenation will consume DO in the river through the consumption of the organic wasteDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Initial Stage: River and Effluent MixingDOa = (DOsVs+ DOpVp) / (Vs+ Vp)where: DOais the DO conc. in the mixed water (mg/L)DOsis the DO conc. in the river water (mg/L)Vsis the volume of river water before mixing (L)DOpis the dissolved oxygen concentration in the effluent (mg/L)Vpis the volume of effluent (L)•For flowing water, we can use flow rates (Q in L/sec) instead of volumes, yielding:DOa = (DOsQs+ DOpQp) / (Qs+ Qp)• We can apply the very same mass-balance principles to calculate the BOD values immediately after mixing the river water and the effluentDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Predicting Downstream DO LevelsInflows•Organic materialenters the river naturally at a constant rateBODin(t) = AOutflows•BOD (and linked DO) is consumed by decompositionBODout(t) = k1BOD(t)•DO (linked to BOD) is consumed by decompositionDOout(t) = k1BOD(t)•DO is recharged by O2from the atm. up to DOsatDOin(t) = k2[DOsat (t) – DO(t)]Biochemical Oxygen DemandDissolved OxygenDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Predicting Downstream DO Levels –Steady-State Conditions• Note that the DO level will never reach DOsatas long as there is some BOD naturally entering the system at rate A, as is usually the case• However, if the reoxygenation coefficient (k2) is a sufficiently large value when compared to A, then the denominator of the term is big enough to make the overall term quite small, which results in the steady-state condition for the DO reservoir only being slightly smaller than the DOsatvalueAk1BOD =Ak2DO = DOsat-David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Greenhouse Gases and Global Warming• What are greenhouse gases (GHG) ?– Amongst the constituents of the atmosphere are some gases that have a particular property: They contribute to the Earth’s ability to retain energy received from the Sun–