Thermal Mediation in a Natural Littoral Wetland: Measurements and Modeling The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Andradóttir, Hrund Ó., and Heidi M. Nepf. “Thermal Mediation in a Natural Littoral Wetland: Measurements and Modeling.” Water Resources Research 36.10 (2000): 2937–2946. ©2000 American Geophysical Union. As Published http://dx.doi.org/10.1029/2000WR900201 Publisher American Geophysical Union Version Final published version Citable link http://hdl.handle.net/1721.1/69012 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. WATER RESOURCES RESEARCH, VOL. 36, NO. 10, PAGES 2937-2946, OCTOBER 2000 Thermal mediation in a natural littoral wetland: Measurements and modeling Hrund0. Andrad6ttirand Heidi M. Nepf Ralph M. ParsonsLaboratory, Department of Civil and EnvironmentalEngineering MassachusettsInstitute of Technology,Cambridge Abstract. As a river flowsthrough shallow littoral regionssuch as wetlands,forebays, and side arms,the temperatureof the water is modifiedthrough atmospheric heat exchange. This process,which we call thermal mediation,can control the initial fate of river-borne nutrient and contaminantfluxes within a lake or reservoir.This paper presents temperatureobservations that demonstratethe occurrenceof thermal mediation and directlysupport the theoreticalresults derived by AndradOttirand Nepf [2000]. The measurementsshow that the wetlandwarms the river inflow by approximately1-3øC during summer and fall nonstormconditions. Less thermal mediation occursduring storms,both becausethe residencetime is significantlyreduced and becausethe wetland circulationshifts from laterallywell mixed (low flows)to short-circuiting(storms). The dead-zonemodel can simulateboth theseregimes and the transitionbetween the regimes and is therefore a good choicefor wetland modeling. 1. Introduction This paper presentsdetailed observationsof thermal medi- ation in a natural wetland that receivesunregulated river in- Wetlands can play an important role in improving down- flow. The major contributionsof the paper are to (1) demon- stream water quality. Numerous massbalance studieshave strate through field observation that wetland thermal shown that suspendedsediments, nutrients, metals, and an- mediationoccurs in a smallwatershed and how it is affectedby thropogenicchemicals are efficientlyremoved in natural and flow conditions,(2) comparewetland thermal structureand constructedwetlands through a variety of sink mechanisms, circulationduring low flowsand storms,(3) validatethe useof suchas bacterial conversion,sorption, sedimentation, natural the dead-zonemodel in wetlands,both by confirmingthe an- decay,volatilization, and chemicalreactions [Tchobanoglous, alytical dead-zone model results derived by Andrad6ttir and 1993].Yet other studieshave shown that wetlandsmay alsoact Nepf [2000] and by showingthat the model simulateswell as a temporalsource of nutrientsand pollutantsas they release wetland thermal behaviorduring variable flow conditions,and storedmaterials [Mitsch and Gosselink,1993, p. 157]. To date, (4) demonstratethe effect of wetlandthermal mediationon extensive research has been conducted to understand the lake intrusiondynamics. This paper is the observationalcoun- chemical,biological, and physicalprocesses underlying the terpart to the theorypresented byAndrad6ttir and Nepf [2000]. sink/sourcepotential of these complexecosystems. However, one processthat so far has receivedlittle attention is thermal mediation,i.e., the temperaturemodification of the water flow- 2. Theoretical Background ing through the wetland. For littoral wetlands the outflow Thermal mediation is a well-known process in cooling temperature determinesthe lake intrusion depth which, in ponds, in which atmosphericheat exchangeis exploited to turn, affects the initial fate of nutrients/contaminants in the attenuatethe waste heat from power plants.Andrad6ttir and lake, aswell as the residencetime and mixingdynamics within Nepf [2000] showedthat thermal mediationis also an impor- the lake. Using a dead-zone model, Andrad6ttir and Nepf tant processin littoral wetlands,when the river inflow follows [2000]showed that wetlandthermal mediation can profoundly a different seasonaltemperature cycle than the wetlandwater. affect intrusiondepth. For example,this processcan shift the This condition is met in small or forested watersheds, where timescaleof lake intrusion depth variability from predomi- the river followsa dampedseasonal cycle because of ground- nantly seasonalto synopticand diurnal. Moreover, wetlands water recharge[Gu e! al., 1996] and/or sun shading[Sinokro! can prolongsurface intrusions during summer, which can lead and Stefan, 1993]. Thermal mediation is less significantin to increasedhuman exposureto river-borne contaminants. largerwatersheds, in which the river has enoughtime to ½quil- Similarly,the increasednutrient supplyto the epilimniondur- ibrat½with the atmosphereand in which sunshading and wind ing the growing seasoncan accelerate lake eutrophication shelteringare lessprominent. [Carmacket al., 1986;Metropolitan Council, 1997]. On the other While the inflow condition sets the potential for wetland hand, more surfaceintrusions lead to quickerflushing, poten- thermal mediation, the degree of thermal mediation actually occurring is governed by the residence time distribution tially reducingthe long-termdeposits of nutrientsand contam- inants in the lake. Wetland thermal mediation can therefore (RTD) withinthe wetlandand the thermalinertia of the water /heat'The thermal inertia representsthe heating timescaleof havecomplex short- and long-termeffects on lake water quality. the water columnand is definedas the ratio of the water depth Copyright2000 by the American GeophysicalUnion. H, and the surface heat transfer coefficient K. The ratio of the Paper number 2000WR900201. nominalresidence time J to the thermalinertia, also called 0043-1397/00/2000WR900201 $09.00 thermal capacity, 2937 2938 ANDRAD(STTIR AND NEPF: THERMAL MEDIATION IN NATURAL LITTORAL WETLAND Table 1. Vegetation Drag Estimatesin the Upper Forebaya mixed reactorwhere the RTD has a large variancearound the meannominal residence time J (Figurela) [Kadlec,1994]. Vegetation Flow Re = UH/v Cfb A During highflows, however, the circulationbecomes inertia- or Yes low O (103) 0.2-2(1, 2) 28-280 jet-dominated(A, •, andFi -2 < 1), andsubstantial short Yes high O (105) 0.008-0.02(2, 3) 1.1-2.8 circuitingoccurs; that is, a large portion of the river flow exits No high O (105) 0.004(4) 0.55 the wetland in much less time than the nominal residence time abeddrag decreases significantly at highReynolds numbers, Re, due J (Figurelb). Boththese regimes, and especially the short- to pronationof vegetationstem. circuiting regime, produce less thermal mediation than the bEstimatesare adaptedfrom (1) Kadlecand Knight [1996, p. 201], ideal plug flow [Andrad6ttirand Nepf, 2000]. (2) Chen[1976], (3) Dunnet al. [1996,p. 54], and(4)Andrad6ttir [1997, pp. 69, 74]. 3. Methods 3.1. Site Description F = J/theat, (1) The Aberjonawatershed is a medium-sizedwatershed (65 km2 surfacearea) located in suburbanBoston, Massachusetts. is a major factor controlling how much thermal mediation Becauseof long term industrial activities the watershed is occurs. For "ideal" plug flow, r = 0 produces no thermal heavily contaminatedwith heavy metals such as arsenicand mediation,whereas r > 3 producesover 90% of the maximum lead which are routinelytransported downstream by the Aber- thermal mediationpossible in the system[e.g., Jirka and Wa- jona River until they are finally depositedin the Upper Mystic tanabe,1980]. Wetland flow regimes,however, are rarely ideal Lake [Solo-Gabriele,1995; •4urilio et al., 1994].Before entering [Kadlec,1994], and water circulation,which governs both the the lake, the river flowsthrough two littoral wetlands,first the skewnessand varianceof the RTD, will generallymodify how largerupper forebay and then the smallerlower forebay (Fig- efficientlythe wetland mediatesthe water temperatures. ure 2a). Both wetlandsare vegetatedwith water lilies and Wetland water circulationdepends on three meteorological submergedcoontail, and their mean water depth rangesbe- forces: wind, which scales on the mean wind shear stressand tween 1.3 and 2.0 m dependingupon season.In comparison, wetland length as •wL; river buoyancy,which scaleson the the lake epilimnionis approximately5 m deep, and the ther- densityanomaly between the inflow and wetland water and moclineis 5 m wide during the summer. waterdepth as Ap#H2; and river momentum per unitwidth, In this paper, we focuspredominantly on the thermal me- pUSH, where Uo is the inflowvelocity and p is the water diation occurringin the upper forebay shownon Figure 2b. density.In addition,the water circulationis influencedby veg- This wetland providesmost of the thermal mediation in this etationdrag, which scales on the friction factor as • p . system,first, becausethe river dischargesinto it, second,be- The relative importanceof vegetationdrag, wind, and water causeits larger size (thus longerresidence time) allowsmore buoyancyto the river inertia canbe summarizedby the ratio of thermal alterations to occur, and, third, because it receives scales: limited return flow from the lake. In contrast, the lower fore- Vegetationdrag parameter[DePaoli, 1999] bay