Lecture 10. Nutrient and BOD Overloading in Fresh Waters

Transcription

1 Last Time Lecture 10 Nutrient and BOD Overloading in Fresh Waters 1.Nutrients and Nutrient Overloading Today 2. Organic Matter ( OM ) discharge and BOD Overloading a. general watershed effects b. point sources DOC content and biological activity. Nutrient overloading to fresh water, as discussed in last lecture can take numerous forms, such as: a. fertilizers b. sewage c. detergents/cleaning agents d. excessive topsoil erosion All 4 forms will increase total organic carbon (TOC) in the environment from fertilization, but sewage discharge also results in TOC increases directly from the OM content of the waste. 1

2 DOC content and biological activity. Organic Matter can be also introduced into the environment as: a. point sources which cause more localized shifts in water quality, at least initially before dispersion. b. dispersed sources which cause watershed-wide shifts in water quality. Variations in Environmental OM Concentrations In general: higher rate of biological productivity (photosynthesis and respiration) = higher natural TOC in an environment. Water soluble components of TOC will dissolve, creating higher DOC where biological productivity is higher. Some pollutant DOC follows the same pattern (i.e., pollutant DOC that results from increased biological activity per unit area of watershed, such as industrial agriculture and urban effluents). 2

3 dispersed OM/Nutrient sources The concentration of natural and pollutive DOC is also inversely proportional to the flow rate of a river. There is a loose correlation between number of human inhabitants in a watershed and DOC concentration. The effects of human DOC loading are worse where low flow rivers traverse heavily populated areas. N The Human DOC load largely comes from two sources: 1. human organic wastes 2. Nutrient wastes. P Note that Nutrient loading of a watershed in also proportional to population 3

4 All DOC, whether natural or from human sources, is partially decomposed by respiration. More often than not it isn t all transformed to DIC, leaving modified DOC behind, which can still place a BOD stress on the environment. Also... The more labile pollutive DOC compounds are reduced in concentration by respirative decomposition (which makes sense, since the biologicaly-mediated digestion of organic matter is used as a waste-control measure in some environments). The more refractory pollutive DOC compounds are not reduced in concentration by respirative decomposition, which causes them to accumulate in the environment. DOC and P loading in the Rhine river and Lake Constance Notice that DOC loading corresponded to an increase in P concentration in lake Constance (in the upper Rhine drainage). This has caused increased P and DOC. Biologically-labile DOC compounds did not increase much over this period of this study even though inert DOC (refractory compounds) increased steadily. Reactive DOC decomposition provides a means for rivers to acquire large concentrations of refractory DOC compounds without others, which can have a large effect on water quality 4

5 Nutrient laden OM overloading of Rhine drainage surface waters led to a pronounced dissolved O 2 depletion during the 1970's. The effect is much like we saw in lakes last time, although the problem was identified and amended before O 2 was depleted to eutrophic levels at this site. excessive topsoil erosion Soils and diverse flora such as occurs in natural forests strongly regulate OM and nutrient output to surface and ground waters of a watershed through biosphere-geosphere cycling. Deforested watersheds lose this ability, so that in addition to enhanced soil erosion, one often finds increased DOC and nutrient loading of local surface water reservoirs in the decade or so after the forest was removed. In this example: DOC increase and associated ph decrease results in large increases of (plant toxic) Al in the same river after deforestation. 5

6 Point source loading of urban wastes into rivers and lakes High BOD wastes containing contaminant levels of nutrients like N and P and/or DOC/POC produces some additional effects that can be predicted using our Redfield ratio stoichiometry and the physics of water flow. This figure gives the schematic the relationship between photosynthesis, respiration and DOx in a lake and a river near a point-source waste outfall. OM point source loading in a lake: point source nutrient loading enhances surface photosynthetic productivity, even when there are significant particulate levels in the waste (particularly if they settle out quickly). This sort of BOD loading also speeds the rate of eutrophism. While there will be some radial distribution of enhanced activity around the point source of waste effluent, this is generally obscured by currents in the lake. 6

7 OM point source loading in a river: Particulates are kept suspended during flow past the point-source. The waters are turbid so BOD loading causes photosynthesis to initially diminish (or cease). Decomposition of the waste releases DIN and DIP into the waters. The nutrient load will cause an algal bloom in the water. This will cause downstream turbidity even if the waste stream isn't high in particulates, with subsequent diminishment of the amount of photosynthesis relative to the unpolluted condition. Downstream of the point source respiration continues unchecked and therefore without photosynthetic replenishment [O 2 ] and pe decreases. If the waste has very high BOD the river can go eutrophic. At some point further down river (once the particulates have settled appreciably) photosynthesis takes over again and can even exceed respiration, casing an upward "bump" in [O 2 ]. 7

8 The biological and chemical effects of point source waste loading of this type can make water treatment for human consumption from this source very challenging. DOx - dissolved O 2 Before understanding the effect of high BOD waste on water quality, let s review the concept of gas saturation. Gas saturation is governed by Henry s law (see week 5). [O 2 (aq)] = (K H O2 )( P O2 ) K H O2 is highly temperature dependent O 2 mg/l= 8.6 at 25 C and 14.6 at 0 C Nomogram for sea level and average barometric pressure. DOx % saturation values can be determined for a given temperature using this nomogram. Draw a straight line between a DOx mg/l value and the water temperature in degrees C. The percent saturation is read where the line intercepts the saturation scale. 8

9 DOx - dissolved O 2 Some terminology related to dissolved oxygen: Definition: Hypoxia is "low oxygen." In aquatic ecosystems, hypoxia occurs when dissolved oxygen falls below 2 mg/l, which is about the lowest level needed for healthy benthic (bottom dwelling) communities. Most organisms living above the bottom, such as fish, need >4 mg/l. Hypoxic areas are sometimes called "dead zones", because only organisms that can live without oxygen (such as anaerobic microbes) live in these areas. Hypoxia is primarily a problem in estuaries, coastal waters, and some freshwater lakes. Definition: Anoxia is a complete lack of oxygen (0 mg/l) The qualitative evolution of DOx (dissolved oxygen) flow a pulse of oxidizable BOD pollutant is depicted in this figure: The simplest quantitative treatment of DOx evolution is the Streeter-Phelps model, originally developed to study sewage effluent plumes in space and time. 9

10 Let s look at a simplified version of the Streeter-Phelps model in this example problem. After mixing of a sewage effluent plume completely with river water, the total organic carbon content of the river water is 6 mg/l. If the ambient temperature is 25º C, will the river water become hypoxic by complete TOC digestion? Solution: The DOx concentration for 100% air saturated water at sea level is 8.6 O 2 mg/l at 25 C. TOC = 6 mg/l If the TOC is algal protoplasm, the Redfield Ratio in mass equivalents tells us 140 mg of O 2 are consumed by complete decomposition of 100 mg of TOC (see box model last Lecture) 6 mg TOC x 140 mg O 2 = 8.4 mg O 2 L 100 mg TOC consumed )O 2 = = 0.2 mg/l = 2.3% saturation. YES, this is Hypoxic 10

11 Why is this overly simplistic? The calculation implicitly assumes the OM will be consumed instantly, but we know that this is not the case. OM degradation will proceed following a reaction rate law. Plus, as degradation proceeds diffusion and mixing in the river will partially replenish the oxygen consumed. Assuming that all of the OM is degradable... If OM degradation (and thus oxygen consumption) is fast, compared to reaeration, then the river will become hypoxic. If OM degradation is slow compared to reaeration then the waters will not become hypoxic. A BETTER CALCULATION. Biological Oxygen Demand (BOD) = amount of O 2 required by bacteria to oxidize readily degradable total organic carbon. Mole BOD: mol O 2 = 1:1 for CH 2 O + O 2 CO 2 + H 2 O Mole BOD: mol O 2 = 1:1.4 for redfield ratio eqn BOD is traditionally measured in a 5 day incubation that measures total oxygen consumption, although change in total organic carbon can be measured instead. The rate of organic matter oxidation in the water follows a 1st order kinetic rate law, so the temporal evolution of BOD can be written as: BOD = BOD 0 (e -kt ) [where 0 indicates the initial value] 11

12 A BETTER CALCULATION. BOD = BOD 0 (e -kt ) [where 0 indicates the initial value] A common application of this equation is evaluation of the impacts of sewage treatment waste water release, where the oxygen consumption-organic carbon oxidation rate constant (k) is typically ~0.2 mg/day. Now we can write a 1-D advection diffusion equation from the evolution of DO with distance down the river (see next slide) do 2 /dt = diffusive O 2 transport - advective O 2 transport - reactions where the reactions include a reaeration term + a BOD term. Let s assume the advection rate in the river is high, so that we can ignore the diffusive term for simplicity s sake. Let s also assume steady-state conditions of flow, TOC discharge and aeration, or else the calculation get s nasty. 0 = -v(dc/dx) + A(O 2 SAT - O 2 ) - kbod A is the reaeration rate constant k is the BOD decay rate constant. If we transform this into the time domain using x = v*t and substitute in the exponential expression for the BOD decay with time, then the solution for this differential equation is: O 2 = O 2 SAT - [k(bod 0 )/(A-k)][e -kt - e -At ] 12

13 Let s estimate O 2 levels for re-aeration at 10%, 1% and 0% of the BOD decomposition rate, such that A ~ 0.02, 0.002, and 0 For 5 days and A ~ 0.02 O 2 = [0.2(6*1.4)/( )][e -0.2*5 - e -0.02*5 ] = 3.6 (not quite hypoxic = 41% saturation) In 10 days... O 2 = 2.2 (hypoxic = 26% saturation) A O 2 A O 2 A O 2 5 days days so given k = 0.2, hypoxia results after 10 days but not after 5. A higher aeration term could perhaps prevent hypoxia entirely. A more labile OM discharge would increase chances of hypoxia A less labile OM discharge would decrease chances of hypoxia Estuarine Eutrophication/Hypoxia The Thames River estuary in London experiences along-stream stratification because competition between river and tidal flows keep waters in this region for long periods of time. The flow regime gives rise to a vertical salinity profile (we will discuss the more common salt wedge salinity profile of the Tees next week). For many years this unfortunate river stagnation location caused a serious water quality decline as pollution from London's population and industries expanded. 13

14 High BOD in effluents to the Thames Estuary causes a DOx "sag" that has existed since the late 19th century, and worsened progressively from 1890 to 1960, allowing waters to go culturally eutrophic. After about 1950 more stringent controls on effluent discharge has reversed the downward trend in quality, and many fish, long absent from the Thames, are now returning. Hypoxia in coastal marine areas Globally, low-oxygen zones are becoming increasingly common in estuaries and the coastal ocean from increased nutrient load (mainly N and P) from human activity. The evolution of hypoxia in the in the Mississippi river delta and Gulf of Mexico in the last 20 years 14

15 Chesapeake Bay Hypoxia Note the very diminished quantity of fish caught in hypoxic zones (green circles). Note the increase in hypoxic areas with time, and their occurrence in the down-current direction from the Mississippi river 15

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