## 2 Materials and Methods
### Chemical Characterization of ESDD  
A quantitative chemical analysis was performed on the ESDD solutions as they play the major role (when in solution) in reducing the insulator electrical surface resistance and making any water layer electrically conductive. The analysis is useful in order to identify the chemical nature of the soluble salts responsible for the ESDD values. The chemical analysis of soluble salts was conducted by using solutions only from the sixth sampling campaign onward (i.e., sixth seventh, and eighth sampling campaigns, see Table 1) as it represented the starting time of the present research study. The solutions of the previous five sampling campaigns (from the first to the fifth) were discarded after ESDD and NSDD measurements.  
In order to chemically characterize the ESDD, a coupled ion chromatography system (Dionex ICS-90 and ICS-2000) was employed to analyze the ionic inorganic fraction. Firstly, samples were homogenized with a 20 min ultrasonic bath. Prior to analysis, the solutions were filtered (0.45 \(\upmu\)m PTFE membrane, 15 mm Syringe Filters, Alltech, Nicholasville, KY, USA) to remove any possible solid particles in suspension that would damage the chromatographic system. The ICS2000 and ICS90 were equipped with an AS3000 Autosampler. Anions (F\({}^{-}\), Cl\({}^{-}\), NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), PO\({}_{4}\)\({}^{3-}\)) were analyzed with a Dionex Ion Pac AG11 (4 \(\times\) 50 mm) and AS11 (4 \(\times\) 250 mm) Guard and Analytical columns in a gradient elution, from 0.1 to 50 mM of KOH (KOH EGC II cartridge) at a flow rate of 0.8 mL/min. The electrical signal of the eluent was lowered by means of a chemical suppressor (Dionex AMMS III 2 mm MicroMembrane Suppressor, regenerant solution: H\({}_{2}\)SO\({}_{4}\), 0.05 M, Fluka 84720). Cations (Na\({}^{+}\), NH\({}_{4}\)\({}^{+}\), K\({}^{+}\), Mg\({}^{2+}\), Ca\({}^{2+}\)) were analyzed with a CG12A-5 \(\upmu\)m and CS12A-5 \(\upmu\)m Guard and analytical columns by an isocratic elution of MSA (methanesulfonic acid, CH3 SO3H, 20 mM, Fluka 64280) at a flow rate of 0.5 mL/min. Also in this case, the signal of the eluent was reduced with a chemical suppressor (Dionex CMMS III 4 mm MicroMembrane Suppressor, regenerant solution: tetrabutylammonium hydroxide, TBA-OH, 0.1 M, Fluka).  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c} \hline  & & \multicolumn{4}{c}{**MR1**} & \multicolumn{4}{c}{**MR2**} & \multicolumn{4}{c}{**MR3**} \\
**Sampling** & \multicolumn{2}{c}{**ESDD**} & \multicolumn{2}{c}{**NSDD**} & \multicolumn{2}{c}{**ESDD**} & \multicolumn{2}{c}{**NSDD**} & \multicolumn{2}{c}{**ESDD**} & \multicolumn{2}{c}{**NSDD**} \\
**Campaign** & \multicolumn{2}{c}{} & & & & & & & & & & & & & & \\ \hline
1 & 0.011 & – & 0.088 & 0.006 & – & 0.060 & 0.005 & – & 0.064 & 0.002 & – & 0.025 & 0.004 & – & 0.044 & 0.003 & – & 0.016 \\
2 & 0.007 & – & 0.139 & 0.003 & – & 0.044 & 0.00  
The quantification was performed using the external standard method. For inorganic ions, single liquid standards were used (1000 g L\({}^{-1}\), Fluka, Sigma-Aldrich, St. Louis, MO, USA).  
The methodology used is described and validated in [8; 13; 14] and presented as a percentage accuracy, over the whole dynamic range of calibration from 100 \(\mathrm{\SIUnitSymbolMicro mL^{-1}}\) to 0.010 \(\mathrm{\SIUnitSymbolMicro mL^{-1}}\), respectively, by 4.8% (Na\({}^{+}\)), 2.8% (NH\({}_{4}\)\({}^{+}\)), 2.7% (K\({}^{+}\)), 3.8% (Mg\({}^{2+}\)), 6.1% (Ca\({}^{2+}\)), 0.7% (F\({}^{-}\)), 0.9% (Cl\({}^{-}\)), 0.9% (NO\({}_{3}\)\({}^{-}\)), 2.7% (SO\({}_{4}\)\({}^{-}\)), and 6.1% (PO\({}_{4}\)\({}^{3-}\)).