How Summer Heat Affects Heavy Metal Levels in Water

Summer heat is not merely a matter of rising temperatures. It brings a cascade of environmental changes—lower dissolved oxygen, increased stormwater runoff, thermal stratification, and enhanced biological activity—that collectively influence the behaviour of heavy metals in water bodies. 

The overall effect can be understood through two main pathways: intensified internal release from sediments and increased external inputs from catchment runoff. These processes are often amplified by biological uptake, making summer a critical period for heavy metal pollution control.

Internal Release from Sediments

Sediments are the primary repository for heavy metals in aquatic systems. During summer, high temperatures transform these sediments from a sink into a source through several mechanisms.

First, elevated temperatures accelerate microbial decomposition of organic matter, which is a major carrier for heavy metals. As organic matter breaks down, the metals bound to it are liberated into pore water and then diffuse into the overlying water column. Second, oxygen depletion near the sediment surface—common in warm, stratified lakes—promotes the reductive dissolution of iron and manganese oxides. Metals such as arsenic, cobalt, cadmium, and lead that were adsorbed onto these oxides are consequently released. 

Third, increased temperatures enhance the dissolution of sulphide minerals, further mobilising associated metals. The combined effect is a significant rise in metal concentrations in bottom waters during summer. Studies have confirmed that under anoxic summer conditions, metals like chromium and cadmium show elevated levels in the water column, and sediment pore water concentrations of several metals increase markedly compared to other seasons.

External Inputs from Stormwater Runoff

Summer storms deliver intense rainfall over relatively short periods, generating substantial runoff that carries heavy metals from urban surfaces, industrial areas, and agricultural land into rivers and lakes. Street dust, roof deposits, and vehicle emissions accumulated during dry periods are efficiently flushed into drainage systems. 

Stormwater discharges can cause sharp, short-term spikes in metal concentrations—sometimes doubling or tripling the influx of lead, copper, and zinc. These external loads can overwhelm the assimilative capacity of water bodies, especially when preceded by dry spells that allow pollutants to accumulate on catchment surfaces.