Constructed Wetlands as Nature-Based Solutions: Resilience to Acid Rock Drainage and climatic seasonality in the Cordillera Blanca, Peru

Glacial retreat, accelerated by climate change, exposes rocks rich in metallic sulphides such as pyrite (FeS₂) to geochemical weathering processes, resulting in Acid Rock Drainage (ARD) which releases H⁺, Fe, SO₄^-2 and trace metals that impact water bodies and ecosystems. This phenomenon has been evidenced in the Cordillera Blanca, where climatic seasonality is characterized by 2 periods, rainy and dry. In this context, Constructed Wetlands (CWs) emerge as Nature-Based Solutions (NbS) designed to mitigate effects of ARD. Although CWs have been extensively studied in acid mine drainages, their performance under seasonal and variable climatic conditions in glacial environments requires research.

In Recuay - Ancash, water quality of Negro river impacted by ARD which feeds a CW at ARD Pilot Treatment Plant was evaluated for 6 months every 2 weeks (rainy and dry periods) by taking in situ measurements and determining acidity, sulphates and heavy metals. In addition, modelling was carried out with different loads applied to size and determine average CW efficiencies.

Results of water quality in the river show higher concentrations in dry period compared to rainy period, where pH: 3.15±0.1 - 3.42±0.1, EC: 489.6±103.0 - 252.0±160.2 µS.cm^-1, TDS: 275.5±63.4 - 121.0±78.4, SO₄^-2: 151.1±27.6 - 92.7±38.6, Fe: 16.8±2.3 - 8.5±3.6, Al: 3.5±0.3 - 2.2±0.7, Ni: 0.07±0.01 - 0.04 ± 0.02, Zn: 0.17±0.02 - 0.11±0.05, Mn: 0.79±0.09 - 0.48±0.20, Mg: 11.8±1.8 - 6.5±2.4, Ca: 17.8±2.2 - 11.5±4.5, Si: 4.3±0.4 - 3.5±0.5 and Na: 2.65±0.36 - 2.00±0.49 in mg.L^-1. Cd, Fe, Mn, Al, Co, Zn, Mg, Si, Sr, Be, Ca and Na showed significant statistical differences (p<0.05) between periods.

Concentration in the CW effluent is: pH: 6.4±0.2 - 6.3±0.1, EC: 234.3±17.8 - 146.9±55.2 µS.cm^-1, TDS: 130.2±33.5 - 70.1±26.7, SO₄^-2: 107.1±23.9 - 72.1±36.2, Fe: 1.3±0.3 - 1.1±0.6, Al: 0.05±0.01 - 0.06±0.01, Ni: 0.004±0.009 - 0.001±0.0, Zn: 0.005±0. 004 - 0.003±0.0, Mn: 1.12±0.11 - 0.83±0.38, Mg: 11.2±2.9 - 8.1±3.3, Ca: 32.7±5.5 - 19.6±11.2, Si: 6.5±0.6 - 5.7±0.8 and Na: 2.76±0.28 - 2.06±0.61 in mg.L^-1 showing that there aren’t significant differences (p<0.05) between periods except for Si and Ca. Modelling results with 2 hydraulic operating loads (0.105 and 0.158 m.d^-1) and residence times (0.079 and 0.118 d) at constant flow suggest that the CW is robust regardless of the hydraulic load. Maximum applied loads were 16.5, 26.9, 3.7, 0.7, 0.015 and 0.047 g.m^-2.d^-1 with average efficiencies of 50.4, 49.9, 90.6, 96.9, 97.9 and 98.9 % for acidity, SO₄^-2, Fe, Al, Ni and Zn, respectively. However, negative efficiencies were observed, primarily for  Mn, Mg, Ca, Si and Na due to anaerobic processes and CW substrate and metal chemistry. In this context, CWs have proven to be a resilient and adaptable solution to climatic seasonality.