Table 2.

Overview of essential biological–diagnostic attributes of faecal indicators and associated genetic targets. The overview shows the ‘big five’: sensitivity, specificity, persistence, resistance, and mobility. Most of the shown attributes can be divided into subcharacteristics. Various methods and specification metrics have been suggested for determination.

Characteristic	Basic definition	Remarks	Methods/metrics
Faecal sensitivity	Occurrence of faecal indicator or genetic target in faecal pollution source(s) to be indicated.	Should be ubiquitous and abundant in targeted pollution source(s). Indicator/genetic target of total faecal pollution in human and vertebrate animal faecal pollution sources (primary enteric habitats). MST indicator/genetic marker only in faecal pollution-source-groups to be indicated (human, ruminant, pig, and so on).	Incidence or (binary) faecal sensitivity (% presence in targeted source; e.g. Ahmed et al. 2009, Farnleitner et al. 2010, Shanks et al. 2010a, b); Abundance in target excreta or wastewater (conc. per volume/mass; Farnleitner et al. 2010, Ervin et al. 2013, Mayer et al. 2018)
Faecal specificity	Nonoccurrence of faecal indicator or genetic target in the pristine environment or nontargeted compartment	Indicators/targets of total faecal pollution should be absent in pristine environments not polluted with faeces. MST indicator/marker also absent in nontargeted faecal pollution sources (e.g. human faecal marker not in ruminant excreta).	False-positive occurrence in pristine habitats (concentration per volume or mass; Vierheilig et al. 2012) (Binary) faecal specificity (% presence in nontargeted source groups; e.g. Shanks et al. 2010a,b, Linke et al. 2021) 25th/75th percentile discrimination metric (25th percentile of target minus 75th percentile of nontarget, log-transformed concentrations (Reischer et al. 2013)
Persistence	Extent of survival (i.e. viability) of indicator or molecular detectability (i.e. nondegraded amenable nucleic acids) of genetic target in the (aquatic) environment.	Persistence varies widely among microorganisms and genetic targets and is influenced by many potential abiotic and biotic ecological factors, such as sunlight, temperature, salinity, grazing, and so on.	T90, T99 [time in days needed for a 1 log10 (T90) or 2 log10 (T99) reduction in indicator or genetic target concentration (Mitchell and Akram 2017)] Decay rate coefficient k of, e.g. a first-order decay model (Chick 1908, Balleste and Blanch 2010)
Resistance	Extent of survival (i.e. viability, proliferation, and infectivity) or molecular detectability (i.e. nondegraded amenable nucleic acids) of indicator or genetic target, respectively, towards chemical substances (e.g. metals, antibiotics) and during technical treatment and disinfection processes.	Resistance varies widely among microorganisms and genetic targets and is influenced by many chemical and physical factors, such as type of chemicals (chlorine, ozone,), concentration and contact time (ct-value), temperature and time in thermal processes, and fluence in UV irradiation.	Inactivation rate and kinetics obtained under carefully controlled conditions (Hoff and Akin 1986) Inactivation rate constants Log reduction of the concentration of microorganisms/pathogens; a measure for the effect of a substance or for the efficacy of the process (Guerrero-Latorre et al. 2016). The log reduction to be achieved for a target is determined by risk assessment.
Mobility	Transport characteristics of the indicator or genetic target in the (aquatic) environment	Mobility is influenced by many factors, such as mass and size of the microorganism/phage, its attachment and aggregation behaviour (electrostatic and hydrophobic forces), its detachment behaviour, as well as the motility of certain microorganisms. Mobility characteristics may change as the microorganism decays.	Sedimentation onto the river bed applies to larger-sized microorganisms (protozoa) or microorganisms attached to sediment (Jiang et al. 2015, Wu et al. 2019) Resuspension of microorganisms attached to the riverbed sediments (Jamieson et al. 2005, Kim et al. 2010, Park et al. 2017) Straining due to microorganism size and aquifer material grain size distribution e.g. (Bradford et al. 2003, Tufenkji et al. 2004) Attachment/detachment (Schijven and Hassanizadeh 2000) Motility (Becker et al. 2004)