761 compact ic

The pH and electrical conductivity of the lake surface water were measured using a pH meter (D-71; Horiba, Kyoto, Japan) and a conductivity meter (ES-51; Horiba), respectively. The additional analyses described below were performed at Chikyu Kagaku Kenkyusho Co., Ltd. (Geo-Science Laboratory, Nagoya, Japan) following the Japanese Industrial Standard (JIS) methods (see Key Resources Table). The concentrations of Na⁺, K⁺, Ca²⁺, and Mg²⁺ were determined using an atomic absorption spectrometer (SOLAAR S Series; Thermo Fisher Scientific). The concentration of NH₄⁺ was measured by indophenol blue absorptiometry (UV–160; Shimadzu, Kyoto, Japan). The concentrations of Cl^–, NO₂^–, NO₃^–, SO₄^2–, and PO₄^3– were determined by ion chromatography (761 Compact IC; Metrohm, Herisau, Switzerland). To determine the total nitrogen concentration, the water sample was digested with potassium persulfate and was analyzed by UV-spectrophotometry (UV–160; Shimadzu). Total phosphorus was determined by standard molybdenum-blue colorimetry following persulfate digestion. Total organic carbon was measured using a TOC Analyzer (TOC-VCPH; Shimadzu).

The concentration of the conservative tracer (bromide anion) and chloride (anion naturally present in the groundwater of the site) in the GW samples taken during the analyses were determined using an ionic chromatograph (Metrohm 761 Compact IC) with anionic chemical suppression, together with a conductivity detector. A Metrosep A SUPP5 5–250 column (25 cm length, 4 mm diameter) as a stationary phase was used and 250 μL of the sample was injected. The mobile phase was an aqueous solution of 3.2 mM of Na₂CO₃ and 1 mM of NaHCO₃ at a flow rate of 0.7 ml min⁻¹. A filtering device (0.45 μm) was included in the sample injection system. If necessary, the GW samples were diluted with Milli-Q water prior to their analysis. Standard solutions of sodium chloride (NaCl, Sigma-Aldrich) and sodium bromide (NaBr, Honeywell Fluka) were used for calibration.
The conductivity of the samples was also measured in the field with a portable pH/conductivity unit (Model 914 pH/Conductometer, Metrohm). Additionally, to get an estimation of the evolution of the test in real time, bromide concentrations were also measured in the field using an ion selective electrode (ISE) of bromides (crystal membrane electrode, Metrohm). Prior to the analysis, groundwater samples were diluted (1:2 in volume) using a TISAB (total ionic strength adjustment buffer) solution of KNO₃ (1 M).