Understanding Microbial Biofilms in Receiving Waters Impacted by Airport Deicing Activities (2014)

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58 C H a P T E R 6 This chapter presents the research team’s suggestions for future research to build on the infor­ mation and insights gained through the current investigations. The suggestions include scopes of work and estimated costs for research in the field and laboratory, as well as further modeling tool development and application. It should be noted that these suggestions are not presented with any assumptions of appropriate­ ness for ACRP or any other specific research organization or entity. Instead, they reflect the research team’s opinions of knowledge and information gaps that need to be addressed in the future. 6.1 Field Investigations In­stream concentrations of readily biodegradable organic material, as reflected in COD or BOD, appear to be the dominant factor for biofilm proliferation and accumulation. Quantita­ tive understanding of this relationship is currently lacking. This lack of understanding prevents the establishment of confident design criteria for deicing runoff systems where controlling or eliminating problematic conditions associated with stream biofilms is a requirement. The field and laboratory results of this study suggest shifts in biofilm community structure with changing nutrient conditions. The literature supports the idea that prolific biofilms that arise following an input of high levels of organic C via deicer fluids represent an ecological shift to copiotrophic (that is, organisms that tend to grow in organic­rich environments) conditions (Upton and Nedwell 1989). What is less clear is how those copiotrophs respond when levels of organic C are reduced to background levels, which under most surface water conditions would be considered oligotrophic. As noted, there is evidence of biofilms storing nutrients, including C. Thus, research is needed that documents the change in bacterial diversity as organic C levels in the water column are increased and decreased. A specific question of interest is whether the time between C inputs is long enough for the populations to shift back and forth. This shift would be airport and season specific. The results of this research would provide a better basis upon which to predict the response of stream biofilms to reduced deicer inputs. To better understand the relationship between COD concentrations and biofilm develop­ ment, it is suggested that additional field surveys be conducted during transition periods when benthic communities are changing to and from a condition of heterotrophic and copiotro­ phic dominance. Monitoring during these periods will provide insight into potential threshold concentrations influencing biofilm community composition. Selected sites should be sampled to expand the existing data set and support evaluation of community changes associated with changing in­stream COD concentrations on both spatial (i.e., upstream to downstream) and temporal (i.e., transition to and from heterotrophic and copiotrophic biofilm dominance) scales. Work Plan for Future Research

Work Plan for Future Research 59 Suggested field research consists of biofilm surveys and the collection of water quality sam­ ples, as follows: • Biofilm surveys should be conducted at three monitoring locations in two receiving streams during the deicing season. Surveys should be conducted at each site every other week over two 5­week monitoring periods: one in the late fall/early deicing season and one in the early spring/late deicing season, for a total of six biofilm surveys at each site. • Water quality samples should be collected at the upstream and downstream sites on each stream. A total of eight sets of samples should be collected: one set during each of the biofilm monitoring trips as well as an additional set 2 weeks before each biofilm monitoring period. In addition, two field quality control samples (one blank and one replicate) should be collected at each airport. Water quality constituents should include COD, total phosphorus, ortho­ phosphorus, nitrate + nitrite, Kjeldahl nitrogen, and ammonia. • Conducting these investigations at the existing streams at MKE and GRR would be advanta­ geous because data collected during those studies would be available to support the research. • The results of the field investigations should be analyzed to characterize the extent and mag­ nitude of biofilm communities that develop at each of the stream stations under different seasonal COD conditions. To the extent possible, tests for statistically significant differences between the stations should be conducted. A technical memorandum should be prepared describing how the experiments were conducted, the results of the data analysis, and the con­ clusions drawn. Suggestions for further research should be included, as appropriate. • Estimated cost for this research is $137,000. This cost includes data analysis and preparation of a simple technical memorandum. • The research could be completed over a 1­year period, beginning in September to ensure capturing a full deicing season. 6.2 Laboratory Studies Task 5 laboratory experimental work determined that phosphorus availability may have a unique ability to influence not only the extent of biofilm accumulation in streams receiving air­ port runoff, but also the bacterial composition of these biofilms. In Task 5 laboratory work, the organisms that became dominant under low P condition were filamentous bacteria, similar to Sphaerotilus natans in colony morphology and overall biofilm appearance. Efforts to limit P as a means of controlling biofilm should take this into consideration as S. natans is often perceived to be an undesirable component of stream biofilms. Further laboratory work is therefore needed to better understand the connection between P availability and biofilm composition. Suggested future laboratory research consists of experiments run in 4 continuous stirred tank biofilm reactors, similar to those used in Task 5. The experiments could be conducted as follows: • Each reactor should be inoculated with a culture derived from airport runoff streams, similar to the culture used in Task 5. • C and N in the reactors should be maintained at levels that are adequate for microbial growth, but P levels in each of the four reactors should be set at 2.5 µg/L, 5 µg/L, 10 µg/L, and 25 µg/L. • The experiments should be run for 6 weeks, with biofilm sampling performed weekly. Biofilm assays should include those performed in Task 5 (HPC, fungi, and chlorophyll a) as well as molecular analyses to differentiate between bacterial species. The results of the laboratory experiments should be analyzed to characterize the microbial communities that develop under each of the conditions of P availability, with tests for significant differences between the four levels. A technical memorandum should be prepared describing

60 understanding microbial Biofilms in Receiving Waters Impacted by airport Deicing activities how the experiments were conducted, the results of the data analysis, and the conclusions drawn. Suggestions for further research should be included, as appropriate. The research could be conducted over a period of approximately 5 months, at an estimated cost of $50,000. 6.3 Modeling Tool Refinement and Application A change in biofilm appearance was observed in the laboratory experiments with fluctuating availability of readily biodegradable organic material. The biofilm model applied during this study considers one type (i.e., profile) of heterotrophic organism that is primarily responsible for the degradation of readily biodegradable organic material. The model can be expanded to account for two or more types of carbon­degrading organisms. In doing so, the competition between organisms that result in the apparent difference in biofilm structure may be evaluated. Future research is suggested to expand the model to define relevant processes and state variables, kinetic expressions, conversion factors, stoichiometric relationships, and diffusivity coefficients. The following steps could be pursued to accomplish this: • Conceptualize processes and state variables (e.g., process 1 consumes readily biodegradable organic matter [SS], oxygen [SO2], and macronutrients, and produces heterotrophic organ­ isms [XH]). • Develop kinetic expressions by operating batch­scale reactors to verify the rate of substrate conversion. This might be done in conjunction with the work described under Section 6.2. • Develop stoichiometric relationships through an energetic analysis (i.e., counting electrons). • Calculate diffusivity coefficients for relevant materials in clean water. Biofilm model calibration and validation requires bulk­liquid chemical analyses. It is sug­ gested that future studies include chemical analyses (e.g., COD, TN, NH3­N, NO2­N, NO3­N, SRP, and TP) of bioreactor influent and effluent streams. More sophisticated analytical meth­ ods, such as florescent in situ hybridization (FISH) and quantitative polymerase chain reaction (qPCR) can be applied to evaluate biofilm mass, and identify the genera and relative abundance of bacteria inside a biofilm. It is suggested that these methods be used to quantify biofilm bio­ mass in terms of type of bacteria inside the biofilm, and their relative location and abundance. With this information, the model may be used to provide a quantitative description of biofilm response to water quality conditions. The suggested future research affiliated with modeling includes the following: 1. Conceptualize an expanded model that accounts for two types of competing heterotrophic organisms that thrive or are suppressed under varying organic loads typical of steams that receive deicer­laden runoff from airfields. 2. Conduct chemical analyses on all laboratory experiments (e.g., COD, TN, NH3­N, NO2­N, NO3­N, SRP, and TP). 3. Evaluate all biofilm samples using FISH and qPCR. The research could be conducted over a period of 1 year, at an estimated cost of $100,000.