The surface of the planet, namely, the space from the vegetation canopy down to the bottom of the aquifer, has been recently recognized as the "Earth’s Critical Zone" (NRC, 2001; Lin, 2010; Brantley et al., 2007). In this zone life-sustaining processes such as photosynthesis, water infiltration, water and nutrient storage, evapotranspiration, decomposition of organic debris, and nutrient cycling occur (Lin, 2010). These processes are driven by complex interactions between the solid rock and the percolating rain water, intruding oxygen, plant root growth and the development and respiration of all sorts of living organisms (Amundson et al., 2007). The result is a heterogeneous natural body that provides life-supporting goods and regulation services. Its study requires delimitation and characterization of its different compartments and continuous measurements of the flows of water, suspended and dissolved matter, and gasses that interconnect them (Rasmussen et al., 2011) (Figure 1).
In order to understand the structure and functioning of the Critical Zone, researchers in several disciplines such as soil sciences, atmospheric sciences, hydrology, ecology, geomorphology, and hydrogeology have worked together combining their specific knowledge and research methods (Brantley, et al., 2007; Lin, 2010; Rasmussen et al., 2011). Some decades ago a worldwide program was created to promote the instrumentation of so called "Critical Zone Observatories" at different locations with contrasting geological and geomorphological settings under different climate and vegetation covers (Anderson et al., 2004). These locations were originally selected at sites with little human influence in order to understand the interrelationships of natural processes within the Critical Zone.
Under natural conditions Critical Zone processes tend to approach a steady-state condition, i.e., a quasi equilibrium, particularly if viewed over millennial time frames (Rasmussen et al., 2011). However, human activities have shown to disturb significantly this steady state, changing the rates of many geospheric processes (Brantley et al., 2006). Therefore, sites that have been impacted by different human activities are currently also incorporated as Critical Zone Observatories. The recognition of the increasing human influence on geospheric processes has driven geoscientists to propose a new geologic epoch, named the Anthropocene, to acknowledge the impact not only of agriculture, but also of the extraction and combustion of fossil fuels, ore extraction, and the environmental release of pollutants, among many other human activities on Critical Zone processes (Crutzen, 2002; Williams et al., 2016).
In order to understand the structure and functioning of the Critical Zone, researchers in several disciplines such as soil sciences, atmospheric sciences, hydrology, ecology, geomorphology, and hydrogeology have worked together combining their specific knowledge and research methods
One example of a region that has been impacted by human activities for centuries is the basin of Mexico. It is a closed watershed that has been inhabited by humans for at least 10,000 years (González et al., 2015). Its temperate climate, abundant water resources, and fertile soils attracted human colonization. But this naturally closed basin has also challenged human settlements since prehispanic times, especially due to recurrent floods in the rainy season. In this paper, first, an historic overview of the artificial drainage of the basin is provided, with attention to its implications on lake dessication and land subsidence, both driven also by population growth and the related groundwater overexploitation for drinking water supply. In a second part of the paper, the effects of the water discharge to the neighboring Mezquital valley are shown. Here the surface run-off, mixed with the untreated sewage effluent of Mexico City, has been used for more than a century for irrigation. Our working group installed here a Critical Zone Observatory. We have investigated the effects of irrigation with untreated sewage effluents in the last 100 years by sampling fields that have been irrigated for different lengths of time, and by installing at one location of the soil and the unsaturated zone devices that allow us to quantify water and gas flows and to measure the quality of the infiltrating water.
Impact of Drainage Activities in the Basin of Mexico
The basin of basin of Mexico is located within the Transmexican Volcanic Belt; it is surrounded by volcanic mountain ranges of up to 5480 meters and its bottom is at an altitude of 2240 meters. After the last glacial maximum, approximately 20,000 years before present (BP), the ice at high altitudes started melting, and a lake formed covering, during the wettest period of the Holocene, an area of 2000 square kilometers (O’Hara and Metcalfe, 1997). The oldest human skeleton that has been discovered in the basin is dated 10,755 +/- 75 years BP (González et al., 2015), evidencing human presence for at least 10,000 years. Agricultural activity started according to findings of grain grinding tools presumably 4,500 years BP (González et al., 2003). Intensified agricultural systems using irrigation, nutrient management, and weed control were implemented at least 2,000 years ago (García Cook, 1985). Among these land use systems were so called "chinampas," which were raised agricultural fields, surrounded by narrow canals, filled up with lake sediments and anchored to the ground by willows planted on their edges (Armillas, 1971; Morehardt, 2012). They were cultivated with multiple crops such as maize, beans, squash, tomatoes, and fruit trees, among many others. With this system inhabitants of the lake gained land and optimized water and nutrient supply to the crops, obtaining several harvests per year (Rojas Rabiela, 1995). Armillas (1971) estimated that approximately 120 square kilometers were covered by this land-use system in the basin of Mexico before the arrival of the Spanish conquerors.
In the basin of Mexico drainage for flood regulation and increasing surface sealing by urbanization triggers land subsidence, enforcing continuously the need to drain with more sophisticated technical tools.
When the Aztec nomadic tribes arrived in the basin of Mexico in the 14th century, this prosperous region was inhabited already by about 250,000 people (Parsons, 1989). The Aztecs had to settle on a small island since there was no free room left at the lake’s shore. A legend states that the Aztecs followed instructions provided by their god Huitzilopochtli to found an empire where they find an eagle sitting on a cactus and devouring a snake. As they happened to find the eagle on an island within the salty Texcoco lake, they built the city of Tenochtitlan at precisely this place, far from fresh water resources and vulnerable to floods. They added land to the lake by expanding the chinampa system, and constructed dams to connect the island with the mainland, as well as to separate the salty water of the northern part from the fresh water in the southern part of the lake, where the lake water was diluted by rain and spring water from the Chichinautzin volcanic mountain range (Birkle et al. 1998). They also built several aqueducts to bring fresh water into their city. Yet, recurrent catastrophic floods destroyed the city in recurrent time intervals of about 50 years (Peña-Santana and Levi, 1989).
After the conquest, the Spanish vice-royal government decided to artificially drain the basin to tackle this problem. Enrico Martínez (Heinrich Mann) (1607–1637) built a first canal, the Tajo de Nochistongo, to connect the Cuautitlán river with the Tula river in the northern part of the basin (Departamento del Distrito Federal, 1975) (Figure 2). A second out-flow was built by Porfirio Díaz (1888–1900), which consisted of a canal (Gran Canal del Desagüe) draining sewage and water run-off from downtown Mexico City through the Tequisquiac tunnel into the El Salado river in the neighboring Mezquital valley. It had originally a capacity of 80 cubic meters per second (m3/s), but subsidence has reduced it to 30 m3/s. In the 1970’s the Emisor Central was built (Departamento del Distrito Federal, 1975). This is a tunnel 6 meters in diameter that conducts by gravity up to 120 m3/s out of the basin of Mexico. It starts north of Mexico City, is 50 kilometers long and ends at El Salto river, an affluent of the Tula river (Gutierrez et al. 1995). A fourth out-flow is currently under construction (Túnel Emisor Oriente). This one will drain the eastern zone of the basin of Mexico, will be 62 kilometers long and have a diameter of 7.5 meters; the capacity will be 150 m3/s (CONAGUA, 2015) (Figure 2).
Due to artificial drainage the surface covered by lakes has diminished substantially over the last four centuries, and particularly during the last eighty years (Figure 3). Currently the lake surface within the basin covers less than 89 square kilometers, most of which dry out during the dry season, or are artificially maintained with effluents from water treatment plants. Lake dessication resulted in gains of agricultural land, particularly in the northern part of the basin, but soluble salts accumulated in the portion of lowest altitude, the former Texcoco lake, forming a 60-square-kilometer salty plain in which only highly salt-tolerant plants grow scarcely (Fernández-Buces et al., 2006). In the dry season dust storms transport large quantities of suspended particles into the city, deteriorating the air quality (Jazcilevich et al. 2015).
In the 1970s large efforts by the Ministry of Agriculture and Hydraulic Resources started to lower the groundwater table by draining, to wash out salts by irrigation, and to introduce a salt tolerant grass (Cruickshank, 2007). These actions have markedly reduced the dust generation (Jazcilevich et al. 2015), but nevertheless, suspended particles are the main air contaminant of Mexico City’s atmosphere during the warm dry season (Díaz-Nigenda et al., 2010).
During the last fifty years accelerated urbanization has sealed most of the surface gained for agriculture by lake dessication (Figure 3). Currently 43% of the southern half of the basin of Mexico is urbanized (INEGI, 2010), which in turn hampers rain infiltration, promotes surface runoff, and renews the need to increase the drainage capacity.
The inhabitants of Mexico City get their drinking water (daily consumption per capita: 220 liter) dominantly from groundwater sources (60.3 cubic meters per second [m3 s-1]). The large water consumption per capita originates from leaks in the water distribution system, which are calculated to be 30–40%. Natural groundwater recharge within the basin is of only 20 to 25 m3 s-1, and is decreasing due to surface sealing and drainage of the surface runoff out of the basin (52 m3/s) (Birkle et al. 1998). In the 1970’s, overexploitation of the aquifer (51.4 m3 s-1) started to show its effects: differential land subsidence (> 9 meters), changes of the groundwater flow direction, decrease of well discharge in the southern part, etc., have been the consequences. To satisfy the constantly increasing demand for fresh water, aqueducts were constructed in the 1970s by which surface water from the neighboring Lerma-Cutzamala basin (16 m3 s-1) is imported into the basin of Mexico (Birkle et al., 1998) (Figure 2).
The Wastewater Irrigation System in the Mezquital Valley
The Mezquital valley is located North of the basin of Mexico and has a predominantly semi-arid climate. Mean annual precipitation varies from 700 millimeters (mm) in the South to less than 400 mm in the North. Rainfall distribution is seasonal; most of it falls from June to October. Mean annual temperature is 18 ºC and potential evaporation 1800 mm. The artificial drainage of the basin of Mexico has provided this area with additional water resources. Large haciendas started irrigating in the 1890s, and in 1912 the use of the drainage water for irrigation was allowed officially. Since then, the irrigated area has been constantly growing, along with the discharge volume (Gutierrez-Ruíz et al., 1995). Currently more than 90,000 hectares are irrigated, administrated by 3 Irrigation Districts (ID) (03-Tula; 100-Alfajayucan, and 112-Ajacuba) (Conagua, 2010) (Figure 2).
Currently, sewage is used without treatment for irrigation. Of great concern is its content of pathogens, which have been quantified in the order of 108 MPN (Most Probable Number) in 100 mL (milliliters) for total fecal coliforms, 107 MPN in 100 mL for Enterococci, and between 6 and 23 helminth eggs per L (Yolanda López Vidal et al., unpublished results). Several studies have investigated the human health and environmental impact of the current practice (Cifuentes et al., 1991/92; Siebe and Cifuentes, 1995; Siebe, 1998; Chávez et al., 2011, Dalkmann et al., 2012). Among the impacts on human health, intestinal helminth infections represented the highest risk of an exposure to raw wastewater. The prevalence of other gastrointestinal infections, as those produced by Entamoeba hystolitica and Gardia lamblia, were only partly related to the exposure to untreated wastewater; poor hygiene implied by poverty conditions in non-irrigated areas of the region determined their prevalence to a larger extent.
Other major concerns are those related to the pollutant load of the irrigation water. Particularly heavy metals have shown to accumulate in the upper 30 cm (centimeters) of the soil, although their concentrations in the water are below maximum permissible limits (Siebe, 1994a). However, since they are adsorbed in the soil’s organic matter, they accumulate over time, so that their concentrations in the soil have increased three- to six-fold when compared to non-irrigated soils in the region (Table 1). In addition, so-called emerging contaminants—i.e., substances, such as several pharmaceuticals, that are not regulated because their toxic effects have not yet been studied—have been measured in the irrigation water in very small concentrations, and these also accumulate in the soils over time (Siemens et al., 2008; Gibson et al., 2007; Dalkmann et al., 2012) (Table 1).
|element/compound||non-irrigated soil||irrigated soil (>80 years)|
|Pb (mg kg-1)||12.3 ±1.4||36.8 ±14.6|
|Cd (mg kg-1)||0.1±0.01||1.2±0.4|
|Cu (mg kg-1)||10.5±2.3||35.3±12.3|
|Zn (mg kg-1)||42.3±9.1||138.6±14.6|
|Sulfametoxazole (µg kg-1)||0.1-0.25||4.0-6.1|
|Ciprofloxacin (µg kg-1)||0.4-0.6||1.0-2.0|
|Carbamazepine (µg kg-1)||0.05||5.4-6.5|
Table 1: Increase in total heavy metal contents (means and standard deviations; n=12) and selected pharmaceuticals (ranges) due to 100 years of irrigation with untreated wastewater in soils (0–30 cm) of the Mezquital Valley. (Data from Siebe, 1994a and Dalkmann et al., 2012).
The soils of the region have good filter and buffer capacities, attributable to their silty clay texture, their neutral to alkaline pH and their medium to large content of organic matter (Siebe, 1994a). These characteristics maintain the pollutants adsorbed to the organic matter and clay particles and hinder the solubility. Thus, heavy metals are adsorbed by the crops only in small concentrations. Yet, our sampling of fields that have been irrigated for different lengths of time indicates that heavy metal content in soils and crops is increasing linearly with time. Predictions indicate that the thresholds of cadmium contents in maize grain for a safe consumption will be surpassed in 150 years, and those in alfalfa in 400 years (Siebe, 1994a). The system is therefore not sustainable in the long term and more strict regulations of the metal loads have to be established.
Nevertheless, farmers appreciate wastewater for irrigation due to its large nutrient content. They achieve yields ranging between 12 and 18 t ha-1 of maize, without needing to add fertilizer. In nearby fields, where maize is produced under rain-fed agriculture, 4 t ha-1 (metric tons per hectare) are seldom achieved, and generally only 2 t ha-1 (Siebe et al., 2016). Wastewater discharge allows them further to plant two crops per year, or to grow fodder crops like alfalfa all year long. In the first half of the 20th century, the Mezquital valley was considered among the poorest regions of Mexico (Rodríguez, 1952), but today irrigation provides an income to more than 35,000 farmers in this region.
Another aspect of long-term wastewater irrigation in the Mezquital valley is the artificial groundwater recharge, which has been estimated in 25 m3 s-1 (BGS, 1998). Since irrigation is performed by overflow, larger volumes of water than those needed by the crops are applied. The excess water infiltrates into deeper layers, beyond the rooting zone, until it reaches the groundwater, which is between less than 2 and up to 40 m in depth. In the 1960s new springs emerged at different parts of lower altitude within the valley, and isotope studies have demonstrated that the water source is made up of more than 90% infiltrated irrigation water (Payne, 1965). Yet, as the soils have a large filter and buffer capacity, the water quality satisfies the drinking water standards (Jiménez and Chávez, 2004), and so it is used after chlorination to supply water to more than 700,000 inhabitants of the valley (Lesser-Carrillo et al., 2011).
In our working group we have studied not only the behavior and fate of pollutants over time, but also the processes of the C and N cycle (carbon and nitrogen) in the Critical Zone.
In our working group we have studied not only the behavior and fate of pollutants over time, but also the processes of the C and N cycle (carbon and nitrogen) in the Critical Zone. We have done this by following the water flow path from the canals, through the rooting zone and beyond, down to the aquifer. We have also collected soil and sediment samples at different depths and measured emissions of the greenhouse gases CO2, N2O and CH4 at the soil surface.
Our results indicate that constant water and nutrient additions have increased biomass productivity, above and below ground. Particularly the belowground biomass, which stays in the fields after harvest, has doubled soil organic matter content after 30 years of irrigation in comparison to rain-fed fields (Chapela-Lara, 2011). Irrigation has also increased the microbial activity (Friedel et al. 2000), and improved notably the soil’s structure (Lueneberg et al., 2018). The increase in soil organic matter improves further the soil’s filter and buffer capacity, i.e., the capacity to adsorb and retain pollutants (Siebe, 1994a). Yet, the proportion of soluble organic compounds also increases with time under irrigation; these compounds and the adsorbed pollutants to them remain mobile and are translocated into deeper depths. It is worthwhile to keep on monitoring their fate, since they might reach the groundwater.
Irrigation also adds between 300 to 727 kg ha-1 (kilograms per hectare) of nitrogen to the fields in one cropping cycle (Hernández et al., 2018) (Figure 4). This is more than maize and especially alfalfa can take up. Particular fodder crops such as rye grass and alfalfa absorb up to 83% of the added nitrogen, evidencing a high nitrogen use efficiency of this agroecosystem. Maize can absorb only around 60% of the added nitrogen; the excess leaches out of the rooting zone in the form of nitrate. We have quantified nitrate-nitrogen losses at 60 to 80 cm depth of up to 108 kg ha-1 under maize and of 31 to 66 kg N-NO3 (nitrogen in form of nitrate) under fodder crops. The soil solution beyond the rooting zone has nitrate-nitrogen concentrations of up to 400 mg L-1(milligrams per liter). Yet, when sampling the groundwater, we have only detected occasionally concentrations of nitrate nitrogen of 60 mg L-1, which is only slightly above the drinking water standard. The latter suggests that in the unsaturated zone, between the rooting zone and the aquifer, an important amount of nitrate is denitrified by microorganisms, most probably to N2 (nitrogen gas) (Hernández et al., 2018), evidencing that this agroecosystem has a self-cleaning capacity.
We have measured at the soil surface that up to 3.44 kg N ha-1 is emitted to the atmosphere in form of nitrous oxide (González et al., 2015). This is a greenhouse gas that has a 250–310-fold larger warming potential than CO2 (Mei et al. 2004). The emissions of N2O (nitrous oxide) are largest under maize (up to 2.98 mg N as N2O m-2 hour-1).
All the results obtained until now are very encouraging to continue monitoring Critical Zone processes in this agroecosystem. Currently, a large wastewater treatment plant is under construction, which start operating intermittently in 2016 (Conagua, 2011). Once operating at full capacity, itwill treat 23 m3 s-1 (cubic meters per second) of urban wastewater from the MAMC by an aerobic biological activated sludge system; during the rainy season it will additionally have the capacity to treat 12 m3 s-1 of surface run-off by advanced physic-chemical treatment. The investment costs are of 751.1 million US dollars, provided to 49% by the federal government and 51% by private investors, and the estimated operation costs are 85.3 million dollars per year (equivalent to 0.12 USD/m3 of biologically treated wastewater and 0.07 USD/m3 of physic-chemically treated wastewater). These costs will be charged to the consumers in the MAMC through their potable water bill.
The government institutions expect that treating the water will reduce health risks, particularly of helminth infections, which are currently recognized as a threat for human health in the area (Cifuentes and Blumenthal, 1991/92), as well as the overall pollutant loads. They also expect that part of the soluble nitrogen and phosphorous will be maintained in the effluents so it can be recycled through irrigation. As the effluents will be chlorinated, vegetables, i.e., larger income crops, could be produced. The reduction of suspended particles in the wastewater will also allow the use of drip irrigation and increase the water-use efficiency.
Critical Zone processes have been modified profoundly by human activities in this region, changing soil, crop, water and air quality, and compromising these natural resources for the next generations.
However, if the treated effluent contains smaller nutrient amounts than those needed by the crops and the soil microorganisms, an enhanced mineralization of the currently accumulated soil organic matter might occur. This would then also release the adsorbed pollutants such as heavy metals, pharmaceuticals, and other organic compounds into the soil solution, increasing their plant up-take, or their mobilization into the groundwater. Chlorination might also affect several crops, particularly those that are sensitive to excess chlorine concentrations. If some organic matter is still contained in the effluent, carcinogenic trihalomethanes will be formed. Drip irrigation will be much less efficient than overflow irrigation in washing out excess soluble salts, which in turn could severely affect crop productivity. And under drip irrigation the recharge of the aquifer will be reduced significantly.
The farmers are very skeptical about the plant initiating operations. They are concerned about an increase of the irrigation cost, or about a reduction of the water supply for irrigation, since they fear that the treated effluent will be used as water supply for population. They also think that the smaller nutrient loads in the water will oblige them to buy additional fertilizer, increasing their production costs (pers. com. José Antonio Maya).
Our working group is currently monitoring fields that have been irrigated for different lengths of time, and we will continue doing this following the changes in the Critical Zone processes due to the change in water quality. The Mezquital valley, therefore, offers a unique opportunity to evaluate the impacts of wastewater irrigation using effluents of different quality. Since water resources are becoming more and more scarce globally, and agriculture is the sector that consumes the largest water volumes worldwide (up to 70 to 90% of fresh water resources, FAO, 2011; UNDP, 2006), the reuse of wastewater for agriculture is currently increasing worldwide (Vo et al., 2014; Chen et al., 2013). Several countries can’t afford an advanced treatment of their sewage effluents. It is therefore urgently needed to provide detailed information on critical zone processes related to wastewater irrigation, as well as to link these with effects on human health. There is a need to know to what extent the pathogen, pollutant, and nutrient loads have to be reduced to minimize adverse effects on human and environmental health. The extent of reduction depends further on the filter and buffer capacity of the soils. If sophisticated water treatment is economically unaffordable, risk management practices need to be tested for their efficiency. These include sanitation practices, like wearing gloves, boots, and mouth protectors, and improving general hygienic practices, like hand washing, as well as agricultural crop management, as growing crops, which are not directly in contact with the wastewater, such as cereals. Another task is to adjust the crop fertilization to the nutrient loads provided by the irrigation. And furthermore, decisions will be needed on whether groundwater formed by infiltrated wastewater can be used as drinking water source.
Human activities have altered the regional water balance since prehispanic times, but particularly during the last 120 years. The construction of technically challenging drainage infrastructure has only partly solved flooding problems in the basin of Mexico; the land subsidence triggered by groundwater overexploitation and impeded recharge due to surface sealing makes it necessary to expand the drainage infrastructure and to import potable water from other basins.
Human activities have altered the regional water balance since prehispanic times, but particularly during the last 120 years.
In the semi-arid Mezquital valley the sewage effluents used for irrigation have improved significantly the agricultural productivity of this semi arid region. They have also altered the trophic status, i.e. the nutrient contents of surface and groundwater. Pollutants are currently filtered efficiently by the soil, but this filter and buffer capacity is limited. The upcoming wastewater treatment is intended to reduce excessive nutrient and pollutant loads and especially the risk of gastrointestinal diseases. However, farmers are skeptical about the effects of changing the water quality. Further monitoring of Critical Zone processes is therefore needed to adapt the current land use system to the new scenario. And in general it would be worthwhile to reconsider if sophisticated technological investments to overcome natural limitations to population growth are the best option for humans inhabiting this region. Maybe it would be more sensitive to stop fighting against nature, and to reconsider adapting to the natural environment and promoting equilibrium conditions, rather than introducing technologies that might promote disequilibra.
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