## 1 Introduction  
Water quality is associated with ecosystem preservation, economic growth and social development [1]. Groundwater (GW) quality is critical to the Philippines' overall water resource; hence, monitoring should be given attention. Population expansion and the acceleration of modernization as well as industrialization have resulted in an increasedwater demand [2]. It is inevitable that quality of both surface water and GW is compromised in areas where economy is in transition and where there is increasing urbanization, industrialization, and agricultural activities [3]. However, due to scarcity of elemental laboratory instruments in the Philippines, the preparation requirements of the laboratory station-based instruments for samples, and the travel time from sampling sites to laboratory stations become a challenge to GW quality monitoring especially to elemental concentration analysis. The population of the Philippines has rice in its regular daily meals. Rice fields and techniques to produce high and good quality yields are among the agricultural areas and programs, respectively, that are being supported by the Philippine government. Calapan City, Oriental Mindoro province in the Philippines, with GW as primary water source, is among the top producers of rice in the country. Rice fields require highly flattened area and good water quality as among the criteria to increase yields with good rice quality. Having the Philippines within the tropical storm belt, Calapan City experienced regular flooding as shown in Figure 1. This condition become a challenge to the water quality for rice fields and for domestic supply. The Philippines is rich in natural resources such as metals and non-metallic minerals [5] with tropical climate where annual rainfall is high. However, anthropogenic activities, due to economic development, population positive growth rate and urbanization, caused unintentional adverse effects to the environment. When the pristine environment is disturbed, and minerals are exposed to oxygen and water, chemical reactions happen. Similar condition takes place during weathering. Having rich in mineral resources, high annual rainfall, large flattened agricultural areas, and GW as primary source of water supply, GW quality monitoring is important. Water quality that has elevated concentration of metals such as arsenic (As), copper (Cu), iron (Fe) nickel (Ni), manganese (Mn), lead (Pb), and other metals known for its toxicity characteristics would have acute and/or chronic adverse effects to human health [6; 7; 8]. Several technologies for measurement of the presence of metals in GW exists such as Inductively Coupled Plasma--Mass Spectrometry (ICP--MS), Inductively Coupled Plasma--Optical Emission Spectrometry (ICP--OES), and Atomic Absorption Spectrometry (AAS). These approaches are laboratory-based and require several days before result  
Figure 1: Flood Hazard Map of Calapan City [4]. of analysis is available. This condition become not suitable for field monitoring and measurements [9] in sub-urban, rural areas, areas where access is a challenge and when time is a primary criterion in the analysis. These conditions require in-situ measurements, and accurate detection become a critical component in monitoring the GW quality. In-situ measurements provide observations on a rapid phase as well as covering the wider areas especially those with difficulty in access. This is in contrast of laboratory--based methods with significant limitations such as expensive instrumentation making limited availability, complex sample preparation and applicability in field conditions [10]. There are on-site detection and monitoring techniques such as electrochemical analysis [10; 11], cyclic voltammetry (CV) [12], anodic stripping voltammetry (ASV) [13], square wave anodic stripping voltammetry (SWASV) [14], electro-chemical impedance spectroscopy (EIS) [15], electrochemiluminescence (ECL) [16] and the use of piezoelectric biosensors [17].