Detailed studies needed to understand nutrient exchange between groundwater and surface water for ensuring better sustainability

A recent study published in the Nature journal brings forth an important yet not sufficiently understood aspect about the exchange of nutrients through groundwater seepage into Oneida lake situated in New York. 

The study, conducted over three years, found a significant contribution of phosphorus from the ground water into the shallow portion of the lakes. 

The study highlighted that the water in the pores of lake sediments (Pore Total Phosphorus) reached up to 100 milligram/litre (mg/l) of which only 10 per cent was inorganic phosphorus which is readily available for uptake by aquatic plants. 

This Pore Total Phosphorus can be released in the lake water and result in eutrophication and damage to aquatic life.

Superimposing findings from New York with Indian conditions 

Delhi-based Centre for Science and Environment (CSE) tried to understand how such a phenomenon would span out in Indian conditions. 

Gopal Krishan, Scientist-E Hydrological Investigation Division at the Roorkee-based National Institute of Hydrology, said that rock phosphate is the major geogenic source of phosphorus often found in the states of Madhya Pradesh and Rajasthan. 

A  study published in 1999 also revealed that Udaisagar lake had high phosphate contents of 186 and 236 μg/l in surface and sub-surface waters respectively. The reason behind this is the presence of phosphate mines found 380-600 metres below the ground. 

Another 2015 media report highlighted the link between illicit dumping of mined waste, groundwater seepage and further contamination of the water bodies through seepage flows in Udaidagar Lakes and the impact on health of the locals.

The 2024 study on Oneida Lake in New York, USA highlighted the need to evaluate the phosphorus which emanates from the groundwater sources apart from the contribution from other surface and anthropogenic sources. The study also claimed that groundwater is a significant contributor of phosphorus loading in lake waters. 

The study underlined that such researches have been rarely conducted as it is difficult to characterise and quantify due to differences in its fluxes because of temporal and spatial differences. 

Also, the chemistry of water changes rapidly owing to its interaction with sediments, pressure conditions, biological activities, oxygen concentration, etc. Also with phosphorus, it is assumed that it binds with soil particles and hence it is rarely looked at. 

Some of the studies have overlooked the concentration of phosphates in the littoral (shallow) zones of the lakes. The study spanned its measurements for three summer years and was made at 13 sites with 384 flow measurements done along 88 km of shoreline of the Oneida lake in New York city in the USA. 

The study found that the Total Phosphorus (TP) (a measure of all forms of phosphates) and Soluble Reactive Phosphorus (SRP) (a measure of orthophosphates or the filterable form of phosphates) was lower during summers but in the near shore it was higher than the deep offshore areas by one to two orders of magnitude.

The residential and wetland areas contributed to lower groundwater seepage flows than the field areas. Wetlands also contributed to the lowest phosphorus influxes. 

The groundwater flow rates increased with rains and the flow rates differed between the different sites measured. 

Also, higher rainfall events were found to be linked with higher leaching out of phosphorus into the lakes. Groundwater seepage was estimated at each site using seepage metres. The study concluded that groundwater seepage is a significant source of phosphorus and more studies should be conducted and resulted in an increase in concentrations of Total Phosphorus in the near shore part of the lake.

Groundwater pollution by phosphorus in India

The main source of phosphorus includes soil and rocks, wastewater treatment plants, agricultural runoff, faulty toilets, animal manure, etc.  

Hoysall N Chanakya, a retired Chief Research Scientist from the Bengaluru-based Indian Institute of Science (IISC), comments that in Indian conditions the phosphorus levels in lakes have been rarely found as high as 100 mg/l under normal conditions. 

In India, phosphorus concentrations do not generally exceed beyond 10-15 mg/l that too are rendered soluble when open waters are exposed to sunlight. Although this study is important to us, there is a need to assess whether SRP travels as fast as it does in this study. 

Indian soils (Red or Black) have a tendency to immobilise SRP into insoluble P as they are iron /calcium rich.  This is the bane of Indian soils, says Chanakya. 

Our soils and agriculture is highly dependent upon imported phosphorus brought in at a very high foreign exchange costs.  Phosphorus in our agricultural and aquatic environments is generally deficient in phosphorus with few exceptions.  SRP is rapidly insolubilised /immobilised in both our agricultural and aquatic environments.  

One needs to examine this narrow window wherein before the immobilisation process, does the SRP exacerbate or cause the problems highlighted in the study.  

Much before the build up of toxic ‘cyanobacteria’ there will be competition with typical green algae such as “Chlorella” in all our water bodies.  Exceptions to this can only cause groundwater pollution.

“The lakes in the study are geographically different from our typical talaabs or kere (irrigation tanks) in peninsular India.  The phosphorus input to such water bodies is neither constant nor very high.  During lake restoration, there is typical exchange of SRP from a variety of sources including ground water seeping upwards under specific conditions,” Chanakya said. 

“A good phosphorus-flow assessment between other phosphorus sources (including groundwater as in this case) would be interesting and important to determine its relevance, if any,” he added.

This study also brings us to the fact that groundwater and surface water interactions are not properly understood but these interactions happen all the time everywhere. While our dependence on groundwater is increasing, we have also encroached and polluted our water bodies- forgetting that it is not in silo’s that we can exist. 

The groundwater recharges our lakes and the lakes act as sponges to further recharge the groundwater. 

The vice versa is also true where groundwater contributes to the base flow of the lakes. 

This also means that the chemicals, elements and nutrients along with the water are being constantly exchanged with the surface and subsurface matrix. Understanding this interaction becomes even more important as we plan restoration of our water bodies and groundwater recharge and the kind of treatment to the wastewater which is generated.

Another important component to this planning is water budgeting which accounts for all the inflows and outflows in a particular area. This does not take into account the entry of various elements and nutrients (such as Arsenic, Uranium, Chlorides, nitrates, phosphorus, etc.) in the water ecosystem. Not only these enter the surface and groundwater bodies through anthropogenic sources but are also found in geogenic conditions. Budgeting these elements and nutrients is also important as these existing above beyond permissible limits can have various effects when used for drinking. 

Sushmita Sengupta, Senior Program Manager, CSE, stated that a recent study conducted by CSE's Water Programme team in the state of Uttar Pradesh on lakes and aquifers shows that these lakes and ponds had been strategically made to maximise groundwater recharge. 

"However most of these lakes and ponds are nothing but receptacles of sewage. Needless to say, these are now acting as a source of contaminating the groundwater reserves and more studies have to be taken up to understand the surface and groundwater interactions and take these rate of exchanges into account while planning groundwater recharge and restoration of lakes and waterbodies," Sengupta explained.