Variations in fluid chemistry and membrane phospholipid fatty acid composition of the bacterial community in a cold storage groundwater system during clogging events

Abstract

In order to monitor the operating mode of the cold storage of the German Parliament (North German Basin) the fluid chemistry and the phospholipid fatty acid (PLFA) composition of the indigenous microbial community have been monitored from August 2006 to August 2009. During this time two periods of reduced injection (clogging events) characterized by Fe precipitates and microbial biofilms in filters occurred in the injection wells impairing the operating state of the investigated cold storage. The fluid monitoring revealed the presence of sufficient amounts of potential C and energy sources (e.g. DOC and ${\text{SO}}_4^{2-}$) in the process water to sustain microbial life in the cold storage. In times of reduced injection the PLFA inventory of the microbial community differs significantly from times of normal operating phases indicating compositional changes in the indigenous microbial ecosystem. The most affected fatty acids (FAs) are 16:1ω7c (increase), 16:1ω7t (decrease) and 18:1ω7c (increase), interpreted to originate mainly from Fe and S oxidizers, as well as branched FA with 15, 16 and 17 C atoms (decrease) most likely representing sulfate-reducing bacteria (SRB). Based on this variability, PLFA ratios have been created to reflect the increasing dominance of biofilm forming S and Fe oxidizers during the disturbance periods. These ratios are potential diagnostic tools to assess the microbiological contribution to the clogging events and to find appropriate counteractive measures (e.g. mechanical cleaning vs disinfection). The correlation between changes in the PLFA composition and the operational state suggests that microbially mediated processes play a significant role in the observed clogging events in the investigated cold storage.

Introduction

In recent years, Aquifer Thermal Energy Storage (ATES) systems have become increasingly important for cooling and heating of buildings by storing natural cold or surplus heat in subsurface formations. For a reliable operation of an ATES, it is important to understand the impact of the geothermal plant on the groundwater system. Problems in the technical plant can occur in terms of corrosion, scaling, and clogging all reducing the geothermal utilizability of the aquifer and the operational reliability of the geothermal plant.

Clogging processes can be classified into three categories: physical, chemical and biological (Baveye et al., 1998). The physical clogging progression is mainly caused by suspended solids leading to the accumulation of the suspended solids in the downhole plant tubes and the formation of a filter cake as well as the blocking of pore spaces in the aquifer (Baveye et al., 1998).

The chemical clogging process is linked to chemical parameters in the respective aquifers such as electrolyte concentration, fraction of organic compounds in the aqueous phase, pH, Eh as well as the mineralogical composition of the solid phase, its surface characteristics, and the chemical reactions (precipitation/dissolution) which can lead to clay-sized particles lodging in the pores (Baveye et al., 1998). Chemical plugging is related to scaling formed by precipitation of e.g. gypsum, carbonates or hydroxides (van Beek, 1989). Worldwide, the most often occurring chemical clogging events in water supply systems are induced by the precipitation and deposition of Fe-oxyhydroxides (van Beek, 1989). Abiotic Fe precipitation in wells can be caused by the contact of anoxic groundwater with atmospheric O₂ as e.g. realized by changing water tables (Van Beek et al., 2009) or by mixing with O₂-containing groundwater.

The biologically mediated clogging is caused by the activity of microorganisms in the aquifer or plant (Ralph and Stevenson, 1995, Potekhina et al., 1999, Rinck-Pfeiffer et al., 2000, Inagaki et al., 2003, Coetser and Cloete, 2005). The clogging material is often found to be slimy layers (Smith and Tuovinen, 1985). These biofilms consisting of bacterial populations surrounded by a thick film of microbially derived extracellular polymeric substances EPS are located at or outside the cell surface (Costerton et al., 1995, Laspidou and Rittmann, 2002). They are highly hydrated and form a matrix keeping the cells together and retaining water (Flemming et al., 2007). The biofilm EPS are composed of polysaccharides, proteins, and nucleic acids as well as microbially produced organic substances or the residue of dead cells (Stoodley et al., 2002). The EPS can trap, bind and accumulate organic material as well as capture suspended solids and inorganic precipitates (Laspidou and Rittmann, 2002, Stoodley et al., 2002). Søgaard et al. (2001) showed that rates of biotic Fe oxidation can be 1000 times faster than for abiotic Fe oxidation and the involved biofilm and its EPS serve as a catalyst for the oxidation/precipitation process as well as preventing re-dissolution of the Fe(III) precipitates (Søgaard et al., 2000).

Furthermore biofouling and biocorrosion of construction materials within the plant caused by microbial biofilms can lead to problems in industrial process water and potable water (Sand, 2003, Ungemach, 2003, Coetser and Cloete, 2005). These phenomena can also cause severe disturbances in the technical equipment of geothermal plants such as pipes, pumps, screens, and heat exchangers leading to substantial operating expense. Additionally, the lifetime of geothermal systems is often limited not only by the wearout of the technical equipment but also by the formation and deposition of scale in the well and in the vicinity of the wells (clogging) (van Beek and van der Kooij, 1982, van Beek, 1989, Ralph and Stevenson, 1995).

Sometimes chemical, physical or biological clogging processes can also occur simultaneously. In these cases their interaction makes it difficult to determine which process has been the initial mechanism and which is the most prevalent.

To date, little is known about the biogeochemical interactions of microorganisms within a geothermal plant and the impact of seasonal changes during different operating modes of the plant on the indigenous microbial community. In the current study, biogeochemical monitoring of the fluid chemistry and bacterial community was conducted for the cold storage of the German Parliament buildings (Reichstag ATES) in Berlin from August 2006 to August 2009. During this time, two periods of reduced injection were observed in the plant. The aim of the study was to investigate the changes in the fluid chemistry and bacterial community with time, especially, related to these events. While microbiological approaches were covered by partners in the project (Lerm et al., 2011), the current biogeochemical approach aims to monitor changes in the bacterial communities using characteristic microbial lipid markers such as phospholipids. These biomarkers, forming a major part of the bacterial cell membranes, are regarded to be indicators for living bacterial communities due to their rapid degradation after cell death (White et al., 1979, Harvey et al., 1986). An advantage of this method is that the phospholipid signal represents the whole indigenous bacterial community and, therefore, covers the compositional changes of the whole bacterial population during the monitoring. For the fluid analysis the focus was placed on selected fluid components being potential electron acceptors (${\text{SO}}_4^{2-}$) and donors (dissolved organic C, DOC) for the indigenous microbial community in the ATES.