The large volume of wastewater remains untreated due to inadequate wastewater treatment facilities and the struggle to regulate it in many countries in the world. But it is being reused as a source of irrigation in urban and peri-urban areas. In many parts of the world, for example, Australia, the United States of America, China, and a few European cities have included wastewater reuse as an important dimension of water resource planning and management. In India, around 80% of all wastewater produced is untreated and discharged onto soils and surface water bodies, and thus leached into the groundwater system.

The use of wastewater for irrigation remains a disagreement among policymakers and researchers. The discharge of untreated or partially treated wastewater and its reuse in agriculture farms as irrigation practices will have negative impacts such as contamination of soils, water pollution, and intrusion in agriculture products.

However, the proper planning and reuse of domestic wastewater for agriculture and other sectors alleviates water pollution, conserves freshwater resources, increases food production, and reduces fertilizer costs. Yet, there are not many studies available that explain wastewater irrigation impacts on the total water circulatory system from soils to water to crops.

The present study ‘Impact of urban wastewater reuse for irrigation on hydro-agro-ecological systems and human health risks: A case study from Musi River basin, South India’ by Shivarajappa et al is an attempt to assess the impact of wastewater use on soil, crop, and water and associated health risks based on exposure risk model. 

The study area is part of the Musi River basin situated on the right bank downstream of Hyderabad city in Telangana. The river receives partially treated municipal sewage mixed up with industrial wastewater from the city in and around Hyderabad city. The river water is then diverted to canals for irrigation from the weirs constructed across river Musi. 

The farmers in the urban and peri-urban areas practice wastewater irrigation. The major crops grown in the area are paddy, para grass, and vegetables. The present study covered four major villages Gowrelly, Thimmaiahguda, Quitbullapur, and Pasumamula covering 100 acres where wastewater is being used for irrigation since 1991.

The river Musi is polluted due to municipal sewage and industrial wastewater that contains high concentrations of nitrogen, phosphorus, and metals such as lead, zinc, and iron. The study focused on the presence of various contaminants (heavy metals) in wastewater circulatory transport systems including surface and groundwater as well as in soil and crops that are irrigated in wastewater agricultural areas situated in the surrounding Musi River to know the extent of pollution.

The study analysed physical (pH, electrical conductivity - EC, turbidity, oil and grease, total suspended solids - TSS), chemical (Zn, Cr, Pb, Mn, Cu and Ni) and biological (biological oxygen demand - BOD, chemical oxygen demand - COD, dissolved oxygen - DO) parameters.

Furthermore, the human health risk assessment model was used to assess the potential of non-carcinogenic and carcinogenic health risks of heavy metals for infants, teens, children, and adults (male and female) via different exposure pathways in agricultural soils, water, and crops prescribed by the United States Environmental Protection Agency (US EPA).

The results of biological, physical, major ions and cations, and heavy metal concentration in water, soil, and crops at a small stretch in the Musi River indicated that all heavy metals and major ions are within permissible limits of wastewater reuse except lead. The high values of lead may be due to the dumping of used electric batteries into soil and water bodies, and another source of lead pollution may be due to automobile exhausts.

The salt content in soils and water is gradually increasing which may create waterlogging and damage to soil structure which can negatively impact agricultural production and its sustainability in the future. Water quality is among the most important environmental challenges related to sustainable development and agriculture, particularly in countries where agriculture is the backbone of the economy. Severe metals and other toxic contaminants endanger water, soil, and agriculture. Once a carcinogenic component reaches the food chain, it is extremely difficult to eliminate and has a devastating carcinogenic impact on humans.

The hazard index and total cancer risk were evaluated from different waters for 4 years for different age groups that are infants, children, teens, males and females. The exposure of heavy metals to human health is proportional to their daily consumption. This study, however, considered consumption through drinking water and dermal adsorption. The human health risk assessment based on heavy metals revealed lower to moderate health risks.

The chloride, magnesium, calcium, sodium, potassium, and phosphorus are within permissible ranges of the Indian Soil Standards. The measured organic carbon (OC) content in the soil samples during the study period ranges from 4100 mg/kg to 12,900 mg/kg. The permissible value of organic carbon (OC) ranges between 5000 mg/kg and 7500 mg/kg. The OC values in 2020 were within the allowable limit of Indian soil standards, remaining all the years the OC values exceeded the permissible limit of Indian soil standards.

Building up higher values of soil carbon can help cut greenhouse gas concentrations in the air and reduce global warming. Organic matter plays a significant role in crop production and soil health by improving the soil's physical, chemical, and biological functions. Increasing levels of organic matter aid in soil structure, water-holding capacity, nutrient mineralization, biological activity, and water and air infiltration rates.

In an overview, the study results indicated the increasing trend of salinity in groundwater, traces of heavy metals in soils and water, exceeding concentrations of nitrate (NO3) and biological contamination. However, the study suggests continuous monitoring of the water and soil quality in wastewater irrigated areas to take remedial actions for sustainable agriculture development and protect ecosystems. The mapping of the subsurface and modelling of solute transport to understand groundwater and associated contamination transport will be attempted in future studies.

Hence, there should be a centralized mechanism for treating domestic and industrial sewage before using it for irrigation. Natural treatment method such as artificial/natural wetlands and reed beds can be used for the removal of contaminants from sewage. The decentralized and operational sewage treatment plants can also be planned for the whole Musi River basin to avoid untreated wastewater disposal to streams and the river course.

The full report is available here