- Overexploiting aquifers increases energy costs and lowers economic productivity by increasing human exposure to geogenic neurotoxins
- Over a 100 year future time-frame estimated revenue from agro-export will be less than the costs this activity imposes on the population
- Investing in water treatment substantially lowers costs of deteriorating water quality
In semiarid agricultural regions, aquifers have watered widespread economic development. Falling water tables, however, drive up energy costs and can make the water toxic for human consumption. The study area is located in central Mexico, where arsenic and fluoride are widely present at toxic concentrations in well water. We simulated the holistic outcomes from three pumping scenarios over 100 years (2020-2120); (S1) pumping rates increase at a similar rate to the past 40 years, (S2) remain constant, or (S3) decrease. Under scenario S1, by 2120, the depth to water table increased to 426 m and energy consumption for irrigation increased to 4 × 109 kWh/yr. Arsenic and fluoride concentrations increased from 14 to 46 ?g/L and 1.0 to 3.6 mg/L, respectively. The combined estimated IQ point decrements from drinking untreated well water lowered expected incomes in 2120 by 27% compared to what they would be with negligible exposure levels. We calculated the 100-year Net Present Value (NPV) of each scenario assuming the 2020 average crop value to water footprint ratio of 0.12 USD/m3. Without drinking water mitigation, S1 and S3 yielded relative NPVs of -5.96 × 109 and 1.51 × 109 USD, respectively, compared to the base case (S2). The relative NPV of providing blanket reverse osmosis treatment, while keeping pumping constant (S2), was 11.55 × 109 USD and this gain increased when combined with decreased pumping (S3). If a high value, low water footprint crop was substituted (broccoli, 1.51 USD/m3), the net gains from increasing pumping were similar in size to those of implementing blanket drinking water treatment.
Plain Language Summary
Groundwater is jointly used by for-profit agriculture and domestic households for drinking water. Although agriculture creates jobs and stimulates investment, preventing the exposure of children to neurotoxins in drinking water generally means a more prosperous future. We calculate falling water tables, rising energy costs, increasing concentrations of naturally occurring neurotoxins, decreasing IQ and earnings for people living in the basin owing to different rates of pumping by agriculture. For the different pumping scenarios, we calculate the increase or decrease in revenue for the agriculture sector. We then calculate the net economic gain from increasing or decreasing pumping rates, growing alternative crops, and treating drinking water to remove the neurotoxins. We found that people’s personal incomes will be ever-more reduced by their exposure to higher concentrations of neurotoxins. The benefits of treating water to remove the neurotoxins are much greater than the costs. Furthermore, increasing pumping rates is only profitable over the long term if it is accompanied by growing much higher value and lower water demand crops than are currently being irrigated in the study area. The most urgent issue, at least from an economic growth perspective, is not limiting pumping but rather treating the drinking water.
*Original study with full-text online at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GH000592
2.3.6.IQ Suppression as Function of As and F Concentrations in Drinking Water
This study focusses on the neurotoxic effect of As and F in child cognitive development as an example of one health and economic impact from exposure. The thresholds at which negative health effects occur and the shape of the dose-response curve for a given neurotoxin are uncertain. A general feature, however, of some well constrained dose-response curves of inorganic neurotoxins (i.e., lead (Pb)) is their inverse shapes (Grandjean & Landrigan, 2006). In such cases, the steepest IQ reductions occur across the low concentration end of the expo–sure scale (Grosse et al., 2002).The neurotoxic effects of As ingestion by children are significant even at low concentrations. But the empirically measured dose-response relationship between chronic exposure to As in drinking water or diet and some measure–ment of cognitive performance ranges considerably (Desai et al., 2020; Grandjean & Landrigan, 2006; Hamadani et al., 2011; Nahar et al., 2014; Rocha-Amador et al., 2007; Rodriguez-Barranco et al., 2013; Signes-Pastor et al., 2019; Vahter et al., 2020; Wasserman et al., 2004, 2014, 2016). The reasons for this variability include (a) different cultural setting and age of children to whom a given intelligence test was applied (WISC-III, WISC-IV, Cambridge Neuropsychological Test Automated Battery, and McCarthy Scales of Children’s Abilities); (b) the chemical form of As ingested; (c) genetic and/or metabolic differences across individuals and groups in their ability to detoxify As; (d) whether drinking water or another biological sample (urine, blood, hair, or toenails) was taken to assess exposure and the specific biochemical form of As measured (MMA, DMA, and total); and (e) other study design factors such as sample size and specific confounding variables included in the model (i.e., mother’s education, household income, quality of home environment, and co-exposure to lead or manganese). In spite of these differences, all studies conclude that As exposure, even at levels near the WHO guideline, have a clear negative impact on child IQ. Longitudinal studies suggest that the effect is irreversible (Vahter et al., 2020; Wasserman et al., 2016).The observed threshold of a neurotoxic effect from As exposure ranges across studies. This threshold can be estimated by studies that regressed a measure of cognitive performance on quintiles of As exposure levels. For example, in a cross-sectional study with 272 9–10 year old school children in Maine, USA, children drinking private well water with As concentrations exceeding 5 ug/L had approximately 6.1 lower IQ points than children drinking less than this amount (Wasserman et al., 2014) (dashed line, Figure 4a). In a longitudinal study with 1,523 10 year old children in MATLAB, Bangladesh, a large decrease (7.2) in the Full Development Score in the WISC-IV test (modified for 10 year old Bangladeshi children) was observed when total urinary As concentrations exceeded approximately 30 ug/L (Vahter et al., 2020). The difference in the thresholds may be partly owed to the exposure levels being much lower in the Maine study than those in the Bangladesh study; however, the magnitude of the effects are similar in both cases.Other studies had the statistical power to constrain a continuous dose-response relationship where As concentra–tions in water or urine are loge-transformed and regressed upon IQ or raw scale point decrements (Equation 13 in Table 2) while controlling for confounding variables. A cross-sectional study with 201 10-year-old children in Araihazar, Bangladesh, found that the regression coefficient, BAs, was approximately -1.65 when loge(As) concentrations in drinking water were regressed upon Full-Scale raw scores (Wasserman et al., 2004). A longitudinal study with 1,700 5 year olds in MATLAB, Bangladesh, observed a BAs of -1.4 only for girls when loge(As) concentration in urine were regressed upon Full-Scale IQ scores corrected for age (Hamadani et al., 2011). In spite of the fact that it only applies to full-scale raw scores, the BAs value of -1.65 was chosen for the model to approximate the reduction in IQ points because this model was continuous and regressed on As concentrations in drinking water (solid line, Figure 4a), and the magnitude of the effect is very similar to several high quality studies reviewed above.A neurotoxic effect of exposure to F in drinking water has been widely reported (Choi et al., 2012). Many of these epidemiologic studies, however, did not control for a wide range of potentially confounding variables. Nevertheless, a pooled meta-analysis on 27 studies reported a 0.44 standard deviation shift to the left owing to “exposure” to high F (Choi et al., 2012). This corresponds to approximately seven IQ points on a normally distributed intelligence curve (Choi et al., 2015). The exposed group was typically exposed to F concentrations ranging from 1 to 4 mg/L but ranged up to 11 mg/L in several studies.To the authors’ knowledge, only one study has published a continuous dose-response curve for exposure to F in drinking water that controlled for confounding variables. This cross-sectional study with 155 children from 6 to 10 years old was conducted 100 km north of our study basin in San Luis Potosi (Rocha-Amador et al., 2007). Fluoride concentrations in that study spanned a similar range to those in our study basin (<0.5–16 mg/L). The current median F concentration in our study area is approximately 1 mg/L (Knappett et al., 2020). Based on the reported value of ?F of ?10.1 (units of loge(mg/L)), no reduction in IQ is predicted based on the current median concentration. However, this coefficient suggests that consuming water at the WHO limit of 1.5 mg/L may lower IQ by four points (Figure 4b). This large effect is supported by a recently published longitudinal study that was performed in Mexico City with 299 mother-child pairs. The authors found that each increase of 1 mg/L increase in creatinine-adjusted F concentration in maternal urine over a threshold of 0.8 mg/L during gestation was asso-ciated with five IQ point reductions in their 6–12 year old children (Bashash et al., 2017). The range of maternal, creatinine-adjusted F concentration in urine was 0.23–2.14 mg/L. Thus, although the question of whether expo-sure to F concentrations less than 4 mg/L in drinking water has neurotoxic effects is debated in the literature, a large body of formal peer-reviewed evidence suggests that it can negatively impact cognitive development during certain windows of susceptibility (Till & Green, 2021). What is even less understood is how geogenic neurotox-ins collectively impact cognitive development. In this study, we assume the effects are independent and can be simply added.Figure 4.Published dose-response curves of IQ reduction owing to exposure to As and F in drinking water. (a) Continuous and step style dose-response curves (Wasserman et al., 2004, 2014). (b) Study by Rocha-Amador (2007). Vertical blue and red dotted lines represent the WHO and Mexican drinking water limits, respectively.
3.2.Childhood IQ and Income for Population Living in the Basin
Irrigation pumping will continue to force communities to install deeper wells in the coming decades. A 2% reduc–tion in lifetime earnings per IQ point decrement was determined (Attina & Trasande, 2013; Grosse et al., 2002). Expected median well As and F concentrations were calculated for the three pumping scenarios (Figures 5fand 5g) (Equation 12 in Table 2). Equation 12 in Table 2 predicts that median As and F concentrations will increase to 45 ug/L and 3.4 mg/L, respectively, by 2120 under S1. The reviewed epidemiological dose-response studies (Hamadani et al., 2011; Wasserman et al., 2004, 2014) suggest that children consuming the present-day median As concentration of 14 ug/L (over half the basin’s population) will experience four IQ point reductions (Figure 5h). Under S1, IQ point decrements owing to As exposure are expected to increase to 6 by 2120 but the pumping scenarios do not have a strong impact on IQ point decrements from As owing to the flattening of the dose-response curve beyond current exposure levels (Figure 4a).
At current median F concentrations of 1 mg/L, only slight reductions in IQs are predicted (Figure 4b). By 2120, however, a wide range of IQ decrements from 4 to 12 points owing to childhood exposure to F concentrations are predicted (Figure 5g). This is caused by the steep dose-response curve (Figure 4b). We summed the separate predicted IQ decrements owing to exposure to As and F. Under S1, the population would be suffering a 19 point IQ decrement by 2120 (Figure 5j). The two lower pumping rate scenarios S2 and S3 would keep the population’s IQ suppressed at approximately 12 and 9 points, respectively.
By the year 2120, owing to their reduced IQs, people living in the basin and drinking water from wells untreated under S1, S2, and S3 are expected to have median incomes of 2,609, 3,211, and 3,464 USD, respectively (Figure 5k). If, however, exposure was mitigated at no cost to the consumers, expected median incomes by 2120 rise to 4,174 USD (RO treatment in Figure 5k).
*Original full-text study online at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GH000592