Table 2.

Criteria for assessing study validity

	Low susceptibility to bias	High susceptibility to bias	Risk of bias type
Study design
Comparison domain	Comparison across both space and time (e.g., BACI)	Comparison across space or time only (e.g., CI, BA)	Selection
Control matching at study initiation (not applicable to BA studies, where control and intervention take place at the same site)	Well-matched control and intervention sites, close in proximity or clearly similar, but without spillover effects	Well-matching not evident, other factors likely to influence difference between treatment and control sites	Selection
Effect modifiers and confounding factors during study period	No or minimal presence of co-interventions, co-exposures or trends present that are likely to differ between groups, baseline environmental conditions similar to intervention period; or if present, confounding factors are accounted for	Co-interventions, co-exposures or trends present that are likely to differ between groups, e.g., differing climate conditions, weather events, or other environmental or anthropogenic changes; and factors are not accounted for	Performance
Groundwater measurements
Measurement similarity and representativeness	Similar and representative measurements across groups, unlikely to differ systematically	Systematic differences in measurements, outcomes not assessed similarly, or measurements unlikely to be representative	Detection
Other
Confounding variables not included in other risk of bias types (e.g., funding source)	No obvious confounding factors, or confounding factors present but accounted for	Confounding factors stated or obviously present but not accounted for	Other