Impact of Regulation on the Water Quality of a Mediterranean River: The Case of the Biobío River

Abstract

Water quality deterioration is a key challenge for sustainability in river basins under high anthropogenic pressures. This study evaluates the evolution of the Water Quality Index (WQI) in the Biobío River Basin (Chile) between 1994 and 2023 in relation to major environmental regulatory milestones, including Law No. 19,300, Decreto Supremo No. 90, the establishment of the Environmental Superintendency (SMA), and the implementation of the Secondary Environmental Quality Standard (NSCA). A temporal analysis of the WQI was conducted using data from stations along the main river course and its tributaries, complemented by a causal loop conceptual model to explore the interactions between regulation, compliance, and water quality. The results indicate initial improvements in WQI values following regulatory milestones, followed in some cases by stabilization or decline associated with reduced enforcement. Case studies, such as the closure of the Inforsa pulp mill in 2013, illustrate differentiated responses to regulatory change. The conceptual model reveals feedback loops linking enforcement perception and compliance behavior. These findings underscore the importance of sustained implementation and institutional capacity to achieve long-term improvements in water quality.

1. Introduction

Water quality deterioration is one of the most significant environmental challenges worldwide. The intensification of human activities, such as agriculture, industrialization, and urban development, and climate change, has placed increasing pressure on aquatic ecosystems, affecting the availability of quality water for various uses [1,2,3,4]. Numerous studies have demonstrated that water pollution negatively affects not only aquatic biodiversity but also human health, food security, and sustainable economic development, making water quality management a key component of the global environmental agenda [1,5].

In Latin America, and particularly in Chile, concerns about water quality have increased in recent decades, as the economic development model has intensified the use of water resources, often without adequate environmental regulation [4,6,7,8]. Until the 1990s, Chile lacked effective instruments for controlling water pollution, a situation that began to change with the enactment of Law No. 19,300, Ley sobre Bases Generales del Medio Ambiente (General Environmental Framework Law) [9] and the subsequent implementation of specific regulations such as Decreto Supremo No. 90 (DS 90) [10], which regulates the discharge of liquid waste into continental water bodies, and the Norma Secundaria de Calidad Ambiental for the Biobío River Basin, established by Decreto Supremo No. 9 [11] (Secondary Environmental Quality Standard for the Protection of the Biobío River Basin). These regulations, together with the institutional strengthening of the Ministerio del Medio Ambiente (Ministry of the Environment, MMA) and the establishment of the Superintendencia del Medio Ambiente (Environmental Superintendency, SMA) and the Servicio de Evaluación Ambiental (Environmental Assessment Service, SEA) [12], are key milestones in the development of a water quality management framework in Chile. Nevertheless, effective access to water resources and the remediation of environmental damage remain ongoing challenges, as evidenced by the judicial practices analyzed by [13] which highlight tensions between existing regulations, their administrative implementation, and judicial interpretation.

The Biobío River Basin, one of the largest and most important in Chile, serves as a paradigmatic example of the challenges associated with water quality management in contexts of intense anthropogenic pressure, receiving inputs from forestry, industrial, agricultural, and urban activities [14,15,16,17,18]. Several studies have documented how these pressures have generated spatial heterogeneity in water quality, with better conditions observed in the headwaters and progressive degradation toward the middle and lower sections of the river [15,16]. Moreover, phenomena such as the drought that affected the region between 2010 and 2022 have exacerbated water quality problems, impacting both biogeochemical cycles and resource availability [14,17].

Tools such as the Water Quality Index (WQI) have been developed to assess and monitor surface water quality in an integrated manner. These indices synthesize multiple physicochemical parameters into a single, easily interpretable metric [19,20,21]. In particular, this study applies the WQI methodology adapted in previous studies of the Biobío River Basin [18], a framework that combines critical parameters such as dissolved oxygen, biochemical oxygen demand (BOD), suspended solids, pH, nitrates, and fecal coliforms, following a multiplicative aggregation scheme that enhances sensitivity to extreme variations in water quality [19].

The main objective of this study is to analyze the impact of environmental regulation, specifically Decreto Supremo No. 90 (Emission Standard for the Discharge of Pollutants into Surface Waters, DS 90), the Norma Secundaria de Calidad Ambiental (Secondary Environmental Quality Standard), and the establishment of the Superintendencia del Medio Ambiente (Environmental Superintendency) as an enforcement authority on the water quality of the Biobío River by evaluating trends and changes in WQI values over time. In doing so, the study aims to contribute to the understanding of the effectiveness of public policies under conditions of intense anthropogenic pressure and climatic variability, recognizing, as noted by [13,22,23], that the mere existence of environmental regulations does not in itself ensure the effective protection of water resources unless accompanied by appropriate mechanisms for enforcement, remediation, and effective judicial access.

2. Materials and Methods

2.1. Study Area

The Biobío River Basin is located in south-central Chile and spans approximately 24,000 km², making it one of the largest and most important basins in the country. The river originates in the Andes Mountains and flows westward into the Pacific Ocean, covering a distance of approximately 380 km. The basin encompasses diverse climatic zones, ranging from humid Andean areas to drier coastal regions, and exhibits a Mediterranean-type climate characteristic of central Chile (rainy winters and dry summers) [18].

The territory supports multiple production activities, including forestry, agriculture, aquaculture, and hydroelectric generation, as well as urban settlements, which exert significant pressure on surface water quality, particularly in the middle and lower sections of the river [14,15,16,17]. To capture the spatial and temporal variability of water quality, historical data from various monitoring stations distributed across the basin were used Figure 1.

2.2. Data Sources

This study used historical water quality data collected from two main sources:

• Centro de Ciencias Ambientales EULA-Chile systematically and consistently collected and processed Water Quality Index (WQI) data for the Biobío River Basin as part of the Biobío River Water Quality Monitoring Program (PMBB) between 1994 and 2017 [18].
• The Dirección General de Aguas (DGA) of Chile, which provided water quality records for the period 2018–2023 [11].

From a spatial perspective, in 2015, the Ministry of the Environment of Chile, through Decreto Supremo No. 9 [11], formally defined the “Surveillance Areas” and “Monitoring Stations” of the Norma Secundaria de Calidad Ambiental (NSCA) for the Biobío River Basin. This structure largely relied on historical data provided by the EULA Center’s program [18], but reorganized the monitoring points under a new regulatory control framework for environmental management.

The dataset includes measurements taken at multiple monitoring stations distributed throughout the basin, covering a range of hydrological conditions and anthropogenic pressures. Historical WQI values were analyzed and correlated with the implementation of key environmental regulations, in order to identify patterns of water quality improvement or deterioration, ensuring continuity and consistency in the analysis of water quality trends, as summarized in

Table 1. Monitoring stations used, data source (EULA/DGA), and recording period.

Station Code	Station Name	Surveillance Area	Data Source	Monitoring Period	Observations *
08307001-3	Biobío River before Llanquén	BI-10	EULA/DGA	1991–2023	Continuous data
08317001-8	Biobío River at Rucalhue	BI-20	EULA/DGA	1991–2023	Continuous data
08334001-0	Biobío River at Coihue	BI-30	EULA/DGA	1991–2023	Continuous data
08390000-8	Biobío River before Gomero junction	BI-40	EULA/DGA	1991–2023	Continuous data
08394005-0	Biobío River before La Mochita Plant	BI-50	EULA/DGA	1991–2023	Continuous data
08394003-4	Biobío River at North Mouth	BI-60-1	EULA/DGA	1991–2023	Continuous data
08394004-2	Biobío River at South Mouth	BI-60-3	EULA/DGA	1991–2023	Continuous data
08333004-K	Bureo River upstream of Biobío confluence	BU-10	EULA/DGA	2018–2023	Continuous data
08323001-0	Duqueco River at Cerrillos	DU-10	EULA/DGA	2018–2023	No data between 1994–2002
08375003-0	Laja River downstream of Antuco Hydropower Plant	LA-10	EULA/DGA	1991–2023	Continuous data
08381013-0	Laja River upstream of Caliboro confluence	LA-20	EULA/DGA	1991–2023	Data available from 2014
08386003-0	Laja River at Laja Bridge	LA-30	EULA/DGA	1991–2023	Continuous data
08352003-5	Malleco River at Malleco Bridge (Route 180)	MA-10	EULA/DGA	2018–2023	Data available from 2014
08344001-5	Renaico River at Renaico	RE-10	DGA	2018–2023	Data available from 2014
08359002-5	Vergara River at Nacimiento	VE-10	EULA/DGA	2018–2023	Continuous data

* Note: The record periods correspond to the years for which water quality data are available at each station. “Continuous data” indicates regular monitoring between 1994 and 2023, with annual or seasonal measurements depending on availability. “Data available since 2014” refers to stations integrated into the monitoring program later or with records available only from that year on. “No data between 1994–2002” indicates periods lacking historical information. This classification aims to clarify the potential discontinuities in the analyzed data series.

.

2.3. Physicochemical Variables and Analytical Methods

The water quality data analyzed in this study included the following physicochemical parameters, selected according to the guidelines of the Secondary Environmental Quality Standard (NSCA) for the Biobío River Basin [11]: pH, temperature, electrical conductivity, dissolved oxygen (DO), five-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), total suspended solids (TSS), nitrates (NO₃⁻), nitrites (NO₂⁻), ammonium (NH₄⁺), orthophosphates (PO₄³⁻), total phosphorus (P_t), total aluminum, total iron, chlorides, sulfates, adsorbable halogenated organic compounds (AOX), and phenolic compounds.

Sampling and analysis procedures adhered to standardized water quality protocols and national technical guidelines established by the Superintendence of the Environment (SMA) and the Ministry of the Environment of Chile to ensure compliance with the NSCA [11].

The selection and analysis of the parameters were based on the local regulatory criteria established by the NSCA for the Biobío River Basin [11]. Parameter values were normalized on a 0–100 scale using normalization factors (Ci), originally defined from historical ranges of the basin and regulatory thresholds, and subsequently combined with the relative importance coefficients (Pi) established by the NSCA and applied by EULA for the past 30 years [11,18,24]. The integration of these values enabled the calculation of the Water Quality Index (WQI), which served as the main indicator for the comparative assessment of environmental quality during the study period.

2.4. Calculation of the Water Quality Index (WQI)

The Water Quality Index (WQI) used in this study follows the methodology developed and adapted by [18], specifically adjusted for rivers in a Mediterranean-type climate, typical of central Chile. The WQI enables the integration of multiple water quality variables into a single value that represents the general condition of water resources. The index was calculated using the weighted arithmetic mean formula, as shown in Equation (1).

$$WQI={\displaystyle \frac{\sum _{i=1}^n(C_i·P_i)}{\sum _{i=1}^n{P}_i}}$$

(1)

where $C_i$ is the normalized value of parameter $i$ (scaled from 0 to 100), $P_i$ is the relative weight assigned to parameter $i$, and $n$ is the total number of parameters included in this calculation.

In our application, $C_i$ and $P_i$ were not estimated or tuned by the authors; they were obtained from the Biobío NSCA [11]. Thus, the 0–100 metric reflects accepted regulatory quality classes, with values adapted to the basin through mathematical rescaling.

Table 2. Parameters and weights used in the calculation of the WQI, including normalization values $\left(C_i\right)$ and relative weights $\left(P_i\right)$ for each variable, used in the WQI, as defined by the Biobío NSCA and the EULA-Chile implementation for Mediterranean-type rivers. Sources: [11,18].

Parameter	$\mathbf{Relative} \mathbf{Weight} \left({\mathit{P}}_{\mathit{i}}\right)$	$\mathbf{Normalization} \mathbf{Factor} \left({\mathit{C}}_{\mathit{i}}\right)$
		0	10	20	30	40	50	60	70	80	90	100
Aluminum (mg/L)	0.1	3.3	1.93	1.62	1.4	1.17	0.72	0.43	0.16	0.09	0.06	0.01
Ammonium (mg N/L)	0.13	0.125	0.11	0.1	0.095	0.075	0.05	0.03	0.025	0.01	0.005	<0.001
AOX (mg/L)	0.1	0.825	0.65	0.5	0.3	0.15	0.1	0.05	0.006	0.005	0.002	<0.001
Chloride (mg/L)	0.1	90	60	45	30	15	10	5	2	1	0.5	<0.05
Fecal Coliforms (NMP/100 mL)	0.16	21,000	15,000	10,000	5000	1000	475	50	30	10	3	<0.5
Conductivity (µS/cm)	0.1	300	180	140	100	75	50	25	15	10	5	<0.5
BOD5 (mg/L)	0.17	12	10	8	6	4	3	2.5	2	1.5	1	<0.5
COD (mg/L)	0.1	30	20	15	10	7	5	3	2	1	0.5	<0.25
Total Phenols (mg/L)	0.1	0.04	0.012	0.008	0.005	0.004	0.003	0.0015	0.001	0.0005	0.0002	<0.001
Total Phosphorus (mg/L)	0.14	0.3	0.27	0.22	0.2	0.17	0.12	0.1	0.07	0.05	0.03	<0.01
Total Iron (mg/L)	0.1	3	2.4	1.7	1	0.75	0.5	0.3	0.2	0.1	0.05	<0.05
Nitrate (mg N/L)	0.1	10	7.5	5	3	2.5	2	1.5	1	0.5	0.25	<0.1
Nitrite (mg N/L)	0.07	0.045	0.03	0.025	0.02	0.015	0.01	0.005	0.003	0.0025	0.001	<0.001
Total Nitrogen (mg/L)	0.1	5	4.5	3.5	3	2.5	2	1.5	1	0.5	0.3	<0.1
Orthophosphate (mg P/L)	0.12	0.3	0.27	0.22	0.2	0.17	0.12	0.1	0.07	0.05	0.03	<0.01
Dissolved Oxygen (mg/L)	0.17	<5	5.6–6.2	5.9–6.8	6.2–6.9	6.5–8.7	6.5–8.9	6.5–8.5	6.5–8.3	6.5–7.8	6.5–7.5	<6.5–7
pH (units)	0.11	<5	5.6–9.2	5.9–9	6.2–8.9	6.5–8.7	6.5–8.5	6.5–8.3	6.5–8.2	6.5–8	6.5–7.8	6.5–7.5
Total Suspended Solids (mg/L)	0.1	150	110	80	55	45	30	20	10	5	2	<1
Sulfate (mg/L)	0.1	180	140	100	80	50	40	30	20	10	5	<1

presents the parameters, their corresponding weights, and the normalization factors used to calculate the WQI [18].

The interpretation of WQI values allows the classification of water quality conditions at each monitoring station into distinct environmental categories. This classification follows the scale proposed by the Centro EULA-Chile [18], taking into account the ecological characteristics of river systems in Mediterranean environments.

Table 3. WQI interpretation categories [18].

Quality Class	WQI Range (%)	Environmental Characteristics	Color
I Very Good	91–100	Excellent quality	Blue
II Good	71–90	Acceptable quality	Green
III Fair	51–70	Contaminated	Yellow
IV Poor	41–50	Heavily contaminated	Orange
V Very Poor	0–40	Excessively contaminated	Red

summarizes the quality categories, their corresponding WQI ranges, and the associated environmental characteristics specific to the Biobío River Basin.

2.5. Data Analysis

The trend analysis of water quality was based on historical Water Quality Index (WQI) values calculated for each monitoring station between 1994 and 2023. For this purpose, biannual WQI time series were developed for each station and represented through line graphs, enabling continuous visualization of water quality evolution over time. This approach facilitated the identification of general patterns of improvement, degradation, or stability across different strategic surveillance areas.

For the integrated evaluation of the basin, representative stations were selected: BI-30, BI-40, BI-50, and BI-60 (also referred to as BI-60-1) as they are located in the lower reaches of the river, providing an integrated view of basin-wide water quality conditions.

To contextualize the observed trends, relevant regulatory milestones were incorporated and graphically represented with vertical reference lines, which facilitated the identification of potential changes in water quality associated with the introduction of these policies.

The temporal comparison also considered:

• The detection of periods of sustained WQI improvement following the implementation of environmental regulations.
• The identification of phases of relaxation or stabilization in WQI values, potentially associated with a decline in the perceived level of “fiscalización” (environmental enforcement), as it is locally referred to in Chile. From this point forward, the term “enforcement” will be used to refer to “fiscalización”.
• The evaluation of differences between mainstem and secondary tributary stations highlighted the effects of localized anthropogenic pressures.

All data preprocessing and graphical outputs were performed using Python version 3.11 within the interactive environment Jupyter Notebook version 2.14.1, with custom scripts developed for WQI calculation, data normalization, and the visualization of time series and regulatory milestones. The corresponding scripts and additional details on data handling and plotting routines are available in the Supplementary Materials.

Additionally, a conceptual analysis approach was employed using causal loop diagrams to visually represent cause-and-effect relationships among key system variables. This tool was used to interpret the interaction between the enactment of environmental regulations, human behavior (such as regulatory compliance or the adoption of more sustainable practices), and the observed water quality response. This perspective enriched the discussion by identifying underlying mechanisms and potential feedback loops that help explain the temporal trends observed in the evolution of the Water Quality Index (WQI).

2.6. Conceptual Modeling

To support the interpretation of the observed evolution of water quality, a conceptual model was developed to qualitatively represent the causal relationships among key system variables. The model was constructed based on a literature review, empirical analysis of WQI time series, and identification of regulatory milestones, following an approach similar to that proposed by [25], which emphasizes the traceability of causal links, diagrammatic clarity, and the usefulness of the model as a foundation for subsequent dynamic modeling efforts.

The model qualitatively represents causal relationships among:

• The enactment and implementation of environmental regulatory instruments.
• The social perception of environmental risk and regulatory enforcement.
• The level of environmental compliance among water users.
• Resulting changes in water quality, measured using the Water Quality Index (WQI).

Within the framework of system dynamics, causal loop diagrams allow the identification of feedback loops that shape system behavior. Two main types of feedback loops exist: reinforcing loops (R), which amplify initial changes, and balancing loops (B), which tend to stabilize the system in response to disturbances [26,27].

This framework was adapted from [25], who proposed a transparent approach for the qualitative representation of complex socio-environmental systems, such as those involving the interactions among regulation, human behavior, and water quality. The conceptual model developed here does not constitute a quantitative simulation but rather depicts key causal relationships intended to support future phases of dynamic modeling [28].

The model integrates three main causal loops: initial reinforcement of compliance triggered by the announcement or enactment of new regulations (R1), sustained reinforcement through visible enforcement once regulations come into effect (R2), and a relaxation loop representing the decline in compliance in the absence of continued enforcement (B1). This structure helps explain the observed patterns of “initial reaction” followed by “subsequent relaxation” in environmental behavior, as reflected in WQI values, a phenomenon widely documented in the international literature on environmental governance in complex social systems [1,2].

3. Results

Given the study’s focus on the descriptive evaluation of temporal trends in the Water Quality Index (WQI), no inferential statistical tests were applied. The analysis focused on identifying visual patterns and structural changes associated with regulatory milestones.

3.1. General Trend of the Water Quality Index (WQI) in the Biobío River Basin (1994–2023)

The historical analysis of the Water Quality Index (WQI) for the Biobío River Basin between 1994 and 2023 revealed a general trend of progressive improvement in water quality (see Figure 2). This evolution was not linear, as it exhibited fluctuations attributable to regulatory and institutional factors, as well as to the behavioral dynamics of regulated actors.

Figure 3 shows the average WQI evolution for the basin, along with the time series data for four key stations located in the lower section of the main river channel: BI-30, BI-40, BI-50, and BI-60. These stations were selected based on data continuity, strategic location, and their relevance for evaluating public water quality policies. Vertical lines indicate major regulatory milestones: the enactment of Law No. 19,300 in 1994 and the publication of Decreto Supremo No. 90 in 2000, which entered into force 180 days after its publication in the Diario Oficial, that is, on 7 March 2001. Notably, its enforcement began on different dates, depending on the type of emission source: for so-called “new sources,” the regulation became enforceable on 3 September 2001, while for “existing sources,” enforcement took effect starting 3 September 2006. This implementation timeline is illustrated in Figure 3 through vertical reference lines corresponding to each regulatory milestone. The figure also highlights the operational launch of the Superintendencia del Medio Ambiente (Environmental Superintendency, SMA) in 2012 and the entry into force of the Norma Secundaria de Calidad Ambiental (Secondary Environmental Quality Standard, NSCA) in 2018.

3.2. Association Between Regulatory Milestones and Responses Observed in the WQI

The temporal relationship between the implementation of regulatory instruments and the changes observed in water quality was systematized. The most relevant findings are as follows:

• 1994 (Law No. 19,300): Marked the beginning of environmental institutionalization in Chile, and initial signs of improvement in the WQI were observed.
• 2000 (Decreto Supremo No. 90): Following its entry into force 180 days after publication in Chile’s Diario Oficial, on 7 March 2001, a sustained improvement was recorded between 2005 and 2008. This trend coincided with the regulatory compliance deadlines for both new emission sources (3 September 2001) and existing sources (3 September 2006).
• 2012–2013 (Operational launch of the SMA): Continued improvement was observed until 2016, which may be attributable to enhanced visible enforcement.
• 2018 (Implementation of the NSCA): A clear recovery in WQI was observed at several stations in response to the establishment of defined quality objectives for the receiving water body.

3.3. Dynamics of Compliance and Relaxation in Response to Regulations

The analysis suggests a recurring pattern of “initial reaction” followed by “subsequent relaxation” in response to regulatory instruments. After the enactment or entry into force of new regulations, a significant increase in WQI values was observed, which was attributable to the compliance efforts of regulated actors. However, in the absence of sustained enforcement, these improvements tended to either stabilize or decline over time.

This behavior aligns with environmental governance theories that emphasize the need to maintain a constant perception of regulatory risk in order to prevent the loss of regulatory effectiveness over the medium and long term.

3.4. Specific Cases: Santa Fe Pulp Mill and Inforsa Plant

The Biobío River Basin has hosted major industrial facilities associated with the forestry sector, notably the Santa Fe and Inforsa pulp mills, both located near the city of Nacimiento along the banks of the Vergara River, one of the main tributaries of the Biobío River. In this context, monitoring station VE-10, located on the Vergara River at Nacimiento, played a key role in assessing the influence of these facilities on water quality.

In addition, the RE-10 and MA-10 stations, located upstream of Nacimiento on the Renaico and Malleco rivers (both tributaries of the Vergara River), provided reference conditions prior to the direct influence of industrial activity. These three stations formed a strategic set for comparing water quality upstream and downstream of the industrial impact zone, enabling a more precise analysis of the local effects of effluent discharges on the fluvial system (see Figure 1 for station locations)

Figure 4 presents Water Quality Index (WQI) values in relation to three key events in this section of the river, which are described below.

• Construction and Operation of the Santa Fe Pulp Mill (2006): Approved through Environmental Qualification Resolution RCA66-2004, the Santa Fe pulp mill began operations in 2006 under the requirements established by Decreto Supremo No. 90. The project was evaluated within the framework of the Environmental Impact Assessment System (SEIA), which mandates verification of regulatory compliance and the implementation of mitigation measures. In this context, the plant incorporated advanced technology for the treatment of RILES (Residuos Industriales Líquidos, or industrial liquid waste), enabling the establishment of high environmental standards from the outset. WQI data recorded at station VE-10 did not show significant deterioration following the plant’s commissioning, suggesting adequate compliance by the company.
• Regulation of RILES from the Nacimiento Sawmill (2013): In 2013, Resolution No. 178 established treatment requirements for liquid waste discharges (RILES) from the Nascimento sawmill. Although the positive impact was more limited than that of other milestones, localized improvement in water quality was expected, which was partially reflected in the data from station VE-10.
• Closure of the Inforsa Plant (2013): This plant, also located in Nacimiento, operated for decades under the name Industrias Forestales S.A. (Inforsa). After entering the Sistema de Evaluación de Impacto Ambiental (Environmental Impact Assessment System, SEIA) in 2005 (Resolution No. 136), the plant ceased operations permanently in 2013. From that year onward, a clear improvement in WQI values was observed at station VE-10, reflecting the positive effect of eliminating a significant source of pollutant discharge.

These cases illustrate how the implementation of specific regulatory measures and the closure of industrial sources had measurable effects on water quality in sections of the Vergara River. They also highlight the importance of maintaining monitoring stations on tributaries, as they allow for the detection of local impacts that may go unnoticed at mainstem stations.

4. Discussion

4.1. Relationship Between Water Quality Trends and Regulatory Milestones

Temporal analysis of the Water Quality Index (WQI) indicates that improvements in water quality in the Biobío River Basin are closely linked to the enactment and implementation of environmental laws, decrees, and regulations in Chile. The upward trend observed particularly following the enactment of Law No. 19,300 [9], the implementation of Decreto Supremo No. 90 (DS90) [10], the operational launch of the Superintendencia del Medio Ambiente (Environmental Superintendency, SMA) [12], and the entry into force of the Norma Secundaria de Calidad Ambiental (Secondary Environmental Quality Standard, NSCA) [11] suggest that regulatory instruments have had a real and positive effect on controlling water pollution.

These findings are consistent with global studies that emphasize the presence of strict regulatory frameworks as one of the most relevant factors contributing to water quality improvement in large river basins [1,2,3,4,20,21].

4.2. Reaction and Relaxation Dynamics in Environmental Compliance

Beyond the enactment of regulations, human behavior in response to environmental policies appears to follow a reaction–relaxation pattern. Following the entry into force of new laws or regulations, an initial improvement in WQI values is typically observed, attributable to increased environmental compliance by regulated actors. However, over time, and in the absence of sustained enforcement (corresponding to the Chilean concept of fiscalización, which in this case consists of the application of the decree, with public personnel going into the field to verify that the decree or corresponding regulations are being complied with), a gradual relaxation in compliance is detected, reflected in the stabilization or decline of water quality indicators.

This dynamic is consistent with the observations of other studies [8], which emphasize that the design of Chile’s environmental regulatory framework has historically faced challenges in ensuring sustained compliance, largely due to institutional gaps in enforcement and sanctioning mechanisms.

In this context, “Reinforcement with Visible Enforcement” refers to periods characterized by visible enforcement actions carried out by the Environmental Superintendency (SMA) and related agencies, including frequent inspections, the imposition of sanctions, and, in specific cases, the suspension or modification of industrial operations (e.g., the closure of the Inforsa pulp mill). In contrast, “relaxation periods” correspond to phases in which enforcement intensity decreased, inspections became less frequent, and perceived regulatory risk was reduced. This distinction helps explain the reaction–relaxation dynamic observed in the WQI time series, reinforcing the link between water quality improvements and the actual implementation of environmental policies in the Biobío River Basin.

4.3. Localized Responses: Industrial Activity and Water Quality Outcomes

Two specific cases provide a detailed view of the relationship between industrial activity and water quality:

• Santa Fe Pulp Mill (2006): Although it was a new source of environmental pressure, its operation under the strict regulatory framework of Decreto Supremo No. 90 prevented significant deterioration in WQI values, indicating successful regulatory compliance during its initial phase.
• Closure of Inforsa (2013): The shutdown of this facility (which had previously operated under limited RILES treatment conditions) resulted in a clear and rapid improvement in WQI values at downstream stations, demonstrating the direct effect of eliminating a persistent pollution source.

These cases confirm that the presence or absence of major industrial sources has a direct impact on water quality outcomes in the basin.

4.4. Importance of Effective Implementation Versus Formal Promulgation

The results indicate that the mere promulgation of environmental laws is insufficient; effective implementation and sustained enforcement efforts are essential for achieving meaningful and lasting improvements in aquatic ecosystems [5]. In the Chilean context, “enforcement” refers specifically to official inspection activities (commonly known as “fiscalización”), such as site visits to verify compliance with environmental laws, decrees, and regulatory standards.

The evidence presented here aligns with the findings of international studies [2], which note that many international water management programs fail not due to a lack of regulation, but because of inadequate mechanisms for enforcement and monitoring.

In this context, the institutional strengthening of agencies such as the Superintendencia del Medio Ambiente (Environmental Superintendency, SMA) has been critical in maintaining the gains achieved following the enactment of environmental regulations in Chile.

4.5. Conceptual Representation: Compliance Cycles in Response to Regulation

To interpret the dynamics observed between the evolution of water quality and the implementation of regulatory instruments in the Biobío River Basin, a conceptual model was developed using a causal loop diagram methodology, as shown in Figure 5. The model integrates three main loops:

• Initial Compliance Reinforcement (R1): This loop captures the initial “reaction” phase following the enactment of new environmental regulations (such as Decreto Supremo No. 90 [10] or Norma Secundaria de Calidad Ambiental [11]. The announcement or implementation of these instruments increases perceived regulatory risk, encourages environmental compliance, and results in short-term improvements in water quality.
• Compliance Reinforcement with Visible Enforcement (R2): This loop extends the “reaction” phase by maintaining a high perception of regulatory risk through sustained and visible enforcement activities (“fiscalización”). This ongoing oversight helps reinforce compliance and supports the persistence of water-quality improvements over time.
• Compliance Relaxation (B1): This loop represents the “relaxation” phase. In the absence of continued enforcement, the perceived risk of non-compliance diminishes, leading to a gradual relaxation of compliance behavior and subsequent deterioration of water quality indicators.

This behavior is consistent with historical experiences in Chile’s water resource governance, where the implementation of environmental regulations has led to initial improvements in water quality, but sustaining these advances has required continuous “fiscalización” and institutional reinforcement [6,7]. Similarly, international literature has documented comparable patterns of initial reaction followed by relaxation in response to environmental regulations in complex social systems [1,2].

The conceptual model suggests that improvements in water quality achieved through regulatory reforms can only be sustained through ongoing and active “fiscalización” efforts, along with continuous reinforcement of public perception regarding environmental risk.

4.6. Implications for Future Environmental Governance

Based on the analysis conducted, the following recommendations are proposed:

• Strengthen periodic enforcement mechanisms and ensure the effectiveness of sanctions.
• Develop environmental risk communication strategies that sustain a high perception of oversight among economic actors.
• Future analyses should consider incorporating natural (climatic and hydrological) and socioeconomic factors, as they may modulate system responses.

Successful environmental governance requires an understanding that ecosystems and human societies are part of the same complex adaptive system [3,22].

4.7. Scope and Limitations of the Interpretative Approach

It is important to note that this study focuses on the observed trend in the temporal evolution of the Water Quality Index (WQI) in the Biobío River basin, with particular emphasis on the relationship between changes in water quality and the promulgation and implementation of regulatory instruments (laws, decrees, and environmental standards). Therefore, this analysis does not consider the potential effects of extreme natural events, such as the 2010 earthquake, prolonged drought periods (2010–2022), or major flood events in 2006 and 2023, even though these phenomena may have influenced water quality dynamics. In particular, drought likely caused a significant reduction in river flows in certain stretches, affecting the dilution capacity and, consequently, the concentration and behavior of pollutants. Likewise, other exceptional global events, such as the COVID-19 pandemic (2020–2022), were excluded. This was a methodological decision aimed at isolating regulatory effects. This event may have indirectly affected water quality in the basin through changes in industrial activity, temporary reductions in environmental enforcement (“fiscalización”), or disruptions in monitoring processes. Although its specific effect was not evaluated in this study, including such factors in future analyses would provide a better understanding of observed variations under conditions of social and economic disruption.

Such circumstances can significantly affect flow volumes in specific river sections, further limiting the system’s capacity to assimilate pollutants, especially in the context of water scarcity. The methodological decision to exclude them reflects the need to isolate and specifically understand human and societal responses to environmental public policies, avoiding the introduction of exogenous variability that could confound the interpretation of the results.

Another limitation relates to the presence of data gaps in certain monitoring stations, as indicated in Table 1. These discontinuities may affect the comparability and reliability of the WQI values over time and space, potentially introducing uncertainty in the detection of trends. Although the integration of records from both the EULA and DGA programs provided a robust long-term baseline, caution is required when interpreting short-term variations, as some apparent changes may reflect monitoring discontinuities rather than actual water quality dynamics.

A further methodological consideration is that the analysis relied on a visual inspection of WQI time series trends, with a specific focus on identifying improvements associated with the implementation of regulatory measures. While this approach is useful for highlighting positive changes, it does not provide the statistical robustness of formal trend tests, breakpoint detection, or regression analysis. Consequently, the results should be interpreted with caution, and future studies should complement this approach with statistical analyses to reinforce the evidence.

In future studies, incorporating a multifactorial analysis that considers both anthropogenic pressures and extreme natural events, along with water resource use conditions, would enable a more comprehensive understanding of the environmental evolution of the basin.

5. Conclusions

This study demonstrates that the evolution of water quality in the Biobío River Basin, as assessed through the Water Quality Index (WQI) over the 1994–2023 period, is closely linked to the progressive implementation of environmental regulatory instruments in Chile. The enactment of Law No. 19,300, Decreto Supremo No. 90, the establishment of the Environmental Superintendency (Superintendencia del Medio Ambiente, SMA), and the implementation of the Secondary Environmental Quality Standard (Norma Secundaria de Calidad Ambiental, NSCA) have had measurable and positive effects on water quality at several monitoring stations.

The results reveal a recurring pattern of initial improvement following the implementation of new regulations, followed by phases of stabilization or regression—particularly in contexts where enforcement (“fiscalización”) has been limited. This reaction–relaxation dynamic, previously documented in complex environmental governance systems [2,22], underscores the importance of maintaining active institutional mechanisms for oversight, sanctioning, and communication of regulatory risk.

Moreover, the analysis of specific cases, such as the closure of the Inforsa pulp mill and the operation of the Santa Fe facility, allowed for the identification of localized impacts in contributing river sections. This highlights the value of complementing basin-wide monitoring with strategically placed stations in critical areas. The conceptual model developed to support a systemic interpretation of findings helps explain the interactions among regulation, perceived risk, compliance behavior, and water quality, aligning with well-established approaches in system dynamics literature [1,2].

It is concluded that the mere existence of regulatory frameworks is not sufficient to ensure sustained improvements in water quality [8]. Effective implementation requires continuous enforcement strategies, institutional strengthening, and integration of system-based analytical tools [20]. To advance toward a more resilient and adaptive water governance framework, future studies should incorporate climatic, hydrological, and socioeconomic factors in line with the multifactorial approaches recommended by [3] for aquatic systems.

This study provides both empirical and conceptual evidence to inform the design and evaluation of environmental public policies in highly pressured basins, such as Biobío, and may serve as a reference for the application of similar models in other regions of Latin America. In addition, the findings of this study make it possible to extrapolate lessons from the Biobío River Basin to other Mediterranean-type and Latin American basins. This experience demonstrates that regulatory frameworks should not be limited to the promulgation of legal standards but must explicitly incorporate continuous enforcement mechanisms in the years following their enactment [5]. The absence of sustained oversight is directly linked to the deterioration of water quality, while its consistent application ensures the persistence of improvements achieved. In this sense, the study underscores the need to complement regulatory reforms with long-term monitoring and enforcement strategies that secure lasting environmental benefits in diverse socio-environmental contexts.