or cost-effective for the foreseeable future, so that identifying short-term mitigation strategies remains essential. The consensus view is that household-level water treatment, dug wells, and rain-water harvesting are not viable alternatives for lowering arsenic exposure because of the cost and logistics of maintaining such systems in rural South Asia (Ahmed et al. 2006; Howard et al. 2006; Johnston et al. 2014; Sanchez et al. 2016). In contrast, despite being the main source of arsenic exposure, tube wells remain the most effective way of providing safe drinking water to the rural population of Bangladesh in the short to medium term (Krupoff et al. 2020). With the exception of the most severely affected areas of the country, the spatial distribution of high- and low-arsenic wells is highly mixed, even over small distances. At the same time, whether a well is contaminated with arsenic or not rarely changes over time (van Geen et al. 2007; McArthur et al. 2010). Therefore, exposure among users of arsenic-contaminated wells can often be avoided by switching to a nearby safe well, be it a shallow private well or a deeper—which usually means safer—community well (van Geen et al. 2002; van Geen et al. 2003). Using data from Araihazar, a sub-district bordering the location of this study, Jamil et al. (2019) estimate that blanket testing campaigns that inform households about the arsenic contamination of all private wells were significantly more cost-effective at reducing arsenic exposure than the provision of piped water, or the construction of deep wells by the government.

Previous campaigns aimed at testing tube well water for arsenic have only partially succeeded at promoting risk-avoiding behavior, highlighting the need to devise novel strategies to achieve this goal. Between 1999 and 2005, the Bangladesh Department of Public Health Engineering (DPHE), with support from the World Bank, DANIDA, UNICEF and a number of non-governmental organizations (NGOs), coordinated the Bangladesh Arsenic Mitigation and Water Supply Program (BAMWSP) testing campaign. The campaign tested close to 5 million wells making use of field kits, and identified them as ‘safe’ or ‘unsafe’—according to the Bangladesh standard of 50 ppb—by painting the well spout with green or red paint, respectively. Several studies have documented switching rates from an unsafe to a safe well after testing of between one-third and three-quarters, with higher switching rates in trials that provided information campaigns on arsenic health risks and repeat visits, in some cases with objective measures of exposure taken in the form of urine samples (Chen et al. 2007; Madajewicz et al. 2007; Opar et al. 2007; George et al. 2012; Bennear et al. 2013; Balasubramanya et al. 2014; Inauen et al. 2014; Pfaff et al. 2017). Despite these partial successes, a substantial fraction of households continues to use unsafe wells today and it is thus important to identify mechanisms to increase riskmitigating responses. In addition, millions of new wells have sprouted in the country, and in most cases users do not know the arsenic level of the water, because campaigns such as BAMWSP have not been replicated, and a market for tests barely exists. There are a few commercial laboratories in Dhaka with the capability to test wells for arsenic, but few rural households are aware of these services.<sup>3</sup> The cost of well testing is greatly reduced and the logistics are greatly simplified by the use of field kits, which have become increasingly reliable and easy to use (George et al. 2012; van Geen et al. 2014), but even these tests are rarely available in the villages.

> 3In addition, the cost of the laboratory analysis is as high as 25-40 USD, not including the cost of the kits necessary for the collection of the water sample.

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