Collaborative environmental governance seeks to engage diverse stakeholders to tackle complex challenges efficiently, sustainably, and equitably. However, mixed empirical evidence underscores a need to understand the conditions under which particularly equity is or is not achieved. Here, we use the empirical case of California Sustainable Groundwater Management to quantify the extent to which vulnerable small and rural drinking water users' needs are addressed in collaborative groundwater planning. Drawing on a diverse array of mixed method data, we then employ Boosted Regression and Classification Trees (BRCT) to assess potential driving factors including collaboration, representation, elite capture, stakeholder engagement, and problem severity/salience. We find each to be influential, highlighting their relevance for equitable planning. We also find evidence that these relationships are complex and outcome specific. Nonetheless, the overall effect on the three equity measures is modest at best. More institutional analysis of collaborative governance regimes from diverse contexts is needed to build a comprehensive understanding of how to meaningfully advance social and environmental equity in such decentralized reforms. Based on our results, we suggest the answer, if there is one, may transcend current focal domains such as stakeholder representation and engagement.

Declining groundwater levels are causing wells to run dry, increasing water and food insecurity, and often acutely impacting marginalized communities across the world. Despite the ubiquitousness and severity of impacts, groundwater research has focused on piecemeal policy instruments or quantified groundwater depletion. How sustainability definitions shape the fate of sustainability reforms and their outcomes is unexplored. Here, we examine one of the world’s large-scale sustainability reforms, the California Sustainable Groundwater Management Act (SGMA), and estimate the impact of sustainability criteria on well failure. We show that proposed sustainability measures accommodate business as usual groundwater level decline, impacting over 10,000 drinking water wells, and 14,000-120,000 domestic well users who could lose access to their source of drinking water. These findings highlight the necessity of carefully and critically engaging in the evaluation of sustainability definitions and their operationalization to prevent detrimental impacts, like threats to household and municipal water supply.