Groundwater Vulnerability Assessment Research Articles

Geohydraulic characteristics and groundwater vulnerability assessment of tropically weathered and fractured gneissic aquifers using combined georesistivity and geostatistical methods

Sustainable groundwater yield in aquifers depends on the protective capacity of the subsurface lithologies and conduit systems. Electrical resistivity tomography (ERT) and its Schlumberger vertical electrical sounding (VES) technique were employed to assess the groundwater yield of aquifer units and their vulnerability to contaminants in Araromi (Akungba-Akoko), southwestern Nigeria. Geohydraulic parameters: aquifer resistivity (\rho0), hydraulic conductivity (K), transmissivity (T), permeability (\Psi), hydraulic resistance (KR), and longitudinal conductance (S) were also evaluated. In addition, regression analysis was utilized to establish the empirical relationships between the K and other geohydraulic parameters, with their percentage contributions to posing vulnerability risk. The georesistivity results revealed four distinct layers: topsoil, weathered layer, partially weathered/fractured bedrock unit, and fresh bedrock. The K model regression-assisted analysis showed that the \rho, T, \Psi, and S contributed about 97.8%, 14%, 99.9%, and 11.5%, respectively, to the estimated aquifers’ K values for the study area. Except for T and S, the regression results had moderate to strong positive correlations with K; hence, this illuminates the essentiality of K in assessing groundwater potential and vulnerability. The aquifer units have low to moderate groundwater yield based on T values (1.67-17.57 m/day) caused by the generally thin overburden (<4 m). However, the deep-weathered and fractured aquifer units with depths ranging from 39-55 m could supply high groundwater yield for sustainable abstraction. The estimated S values (0.0226-0.1926 mhos) for aquifer protective capacity ratings rated the aquifer units in the area as poor to weak. Based on the estimated low logarithm of KR (log KR) values of 0.79-2.25 years, these aquifers have extremely high to moderate aquifer vulnerability index. As a result, prospective wells/boreholes in the study area and settings with similar geohydraulic and vulnerability characteristics should be developed adequately to prevent the infiltration of surface contaminants for potable groundwater abstraction.

A Modified GALDIT Method to Assess Groundwater Vulnerability to Salinization—Application to Rhodope Coastal Aquifer (North Greece)

Aquifer overexploitation in coastal aquifers has led to seawater intrusion that causes severe salinization effects on the groundwater system. The most widespread method for assessing groundwater vulnerability to seawater intrusion, the dominant cause of salinization in coastal aquifers, is the GALDIT method, with numerous applications globally. The present study proposes a modified version of the GALDIT method (GALDIT-Ι) to evaluate the vulnerability of salinization, including its potential additional sources. Both methods have been applied to Rhodope coastal aquifer, an intensively cultivated agricultural area subject to multiple salinization sources. The basic modifications of the proposed GALDIT-I method include different weighting factors and modification of classes for critical parameters, the use of a different indicator (TDS) for the estimation of the Impact factor and, overall, the address of the concept of groundwater salinization instead of seawater intrusion only. The differences in the results of the two methods were significant, as the modified version exhibited a more finite and realistic vulnerability capture, according to the area’s existing hydrogeological and hydrogeochemical knowledge. The original GALDIT method showed an area of nearly 80% as medium vulnerable with very limited spatial deviations. On the other hand, the proposed modified GALDIT method depicted high vulnerability hotspots away from the shoreline, indicating various salinity sources. The validation of the modified method showed that nearly 80% of the sampling points present very good to perfect match between the salinity assessment and the concentration of Cl−, indicating the successful validation of the method. Overall, the GALDIT-I method facilitated groundwater vulnerability assessment to salinization more accurately and exhibited a more discrete spatial assessment, thus, it could be regarded as a promising proactive tool for groundwater management and decision-making.

The Use of the DRASTIC-LU/LC Model for Assessing Groundwater Vulnerability to Nitrate Contamination in Morogoro Municipality, Tanzania

Groundwater is a useful source of water for various uses in different places. The major challenge in the use of this resource is how to manage and protect it from contamination. The current study was conducted in Morogoro Municipality to identify vulnerable groundwater areas by using DRASTIC-LU/LC model. The study applied eight input parameters, i.e., depth to water table, net recharge, aquifer media, soil media, topography, impact of vadose zone, hydraulic conductivity and land use/land cover patterns, which were overlaid in GIS to generate groundwater vulnerable map. The model used rating (R = 1–10) and weighting (W = 1–5) techniques to assess the effect of each parameter on groundwater contamination. The DRASTIC-LU/LC Vulnerability Index map was classified into low- (area = 29.2 km2), moderate- (area = 120.4 km2) and high-vulnerability zones (area = 124.4 km2). Nitrate analysis was conducted using the cadmium reduction method (DR 890) to assess the validity of the model and it was observed that 55%, 15% and 50% of the samples with unacceptable (>50 mg/L), high (29–50 mg/L) and moderate (14–28 mg/L) nitrate concentrations, respectively, fall into the high-vulnerability zone. Furthermore, 45%, 70% and 50% of the samples with unacceptable, high and moderate nitrate concentrations, respectively, fall into the moderate-vulnerability zone. In the low-vulnerability zone, only 15% of samples were found with a high nitrate concentration.

Comparison of DRASTIC and DRASTICL groundwater vulnerability assessments of the Burdekin Basin, Queensland, Australia

In the Burdekin Basin, Queensland, Australia, groundwater contamination due to agricultural activities has led to concerns over its impacts on globally significant ecosystems such as the Great Barrier Reef. An appropriate method for groundwater vulnerability assessment is essential for the sustainable use of this groundwater resource and its longer-term environmental management. The aim of this study is to apply and assess the suitability of the standard DRASTIC index-based method for groundwater vulnerability assessment of the Burdekin Basin. The intrinsic groundwater vulnerability is calculated in ArcGIS, using data for the period 2010 to 2021. The results are compared to available water quality data. The calculated DRASTIC scores are found to be only weakly correlated with water quality parameters, including the nitrate concentration (R = 0.07), which should behave as a proxy measure of groundwater vulnerability. To address this, a modified DRASTICL method containing a land use parameter is also implemented, to assess the specific groundwater vulnerability. The correlation between DRASTICL scores and nitrate levels (R = 0.2) is more significant but is still relatively weak. From this study, it is recommended that alternative methods be developed to assess groundwater vulnerability in the Burdekin Basin, and other comparable aquifer systems.

Comparative assessment of groundwater vulnerability using GIS-based DRASTIC and DRASTIC-AHP for Thoothukudi District, Tamil Nadu India.

The groundwater is very precious in the world. Rapid urbanization and industrialization create tremendous stress on groundwater quality and quantity. Unscientific groundwater extraction and waste disposal methods impact the groundwater aquifer's susceptibility in the coastal area. This research examines how industrial waste, seawater intrusion, and solid waste dumping affect the Thoothukudi District, located on the southwest coast of Tamil Nadu, India. The groundwater vulnerability potential is determined using the DRASTIC and analytical hierarchy process (AHP)-based DRASTIC model. DRASTIC-AHP method's weights and ranks are determined using multi-criteria decision analysis (MCDA)-based pairwise comparison method. Remote sensing (RS) and geographical information system (GIS) are implemented to prepare the input layers for DRASTIC and DRASTIC-AHP. The findings reveal a very high category of vulnerability along the coastline that is covered in sand and loose sediments from an aquifer. Similar conditions exist on the southeast side, which is covered with gravel, sand, and sandstone with shale and has relatively low-slope topography. This enables higher contaminant percolation into the groundwater and raises the possibility for pollution. The DRASTIC-AHP method's results reveal that the southeast side has a significant possibility of contamination. The water table, net recharge, vadose zone, and conductivity greatly impacted the DRASTIC vulnerability assessment due to their stronger weight than theoretical weight. It may be stated that the DRASTIC technique is more cost-effective and time-efficient in analyzing a wide range of regional groundwater risks while avoiding sloppy, uncontrolled land development and other unwanted activities.

Assessment of Groundwater Vulnerability to Contamination using GIS-based DRASTIC model in Lucknow, India

Abstract: Groundwater intrinsic vulnerability maps are considered a promising means for forestalling groundwater resources from contamination in recent years. With the use of a Geographic Information System (GIS) and hydro-geochemical data, the DRASTIC model is used to forecast groundwater vulnerability. For which ArcGIS software is used. Lucknow, capital of Uttar Pradesh, is the study area for this study. This model is based on the seven data layers that provide the input to the modelling. It corresponds to the initials of seven layers i.e. Depth to water, net Recharge, Aquifer media, Soil media, Topography, Impact of vadose zone ,and Hydraulic Conductivity. Mal and Sarojani Nagar Block showed higher vulnerability (139–143). The results also suggested that the low, moderate, and high groundwater vulnerability zones cover around 30.35%, 45.02%, and 24.63 % of the study area, respectively. The results is validated using nitrate concentration in groundwater. The percentage of study area for nitrate concentrations <45 mg/l, 45-60 mg/l, 60-80 mg/l, and 80-110 mg/l are 55.5%, 34.5%, 9%, and ~1% respectively. The GIS technique has provided an efficient tool for assessing and analysing the vulnerability to groundwater pollution.

Groundwater Vulnerability of Halabja-Khurmal Sub-Basin Using Modified DRASTIC Method

Evolving groundwater vulnerability from DRASTIC to modified DRASTIC methods helps choose the most accurate areas that are most delicate toward pollution. This study aims to modify DRASTIC with land use and water quality index for groundwater vulnerability assessment in the Halabja-Khurmal sub-basin, NE/Iraq. The Halabja-Khurmal sub-basin groundwater vulnerability index is calculated from nine hydrogeological parameters by the overlay weighting method. As a result, 1.3% of the total area has a very high vulnerability value and 46.1% with high vulnerability. The regions with high groundwater vulnerability have a high-water table and groundwater recharge. Nitrate concentration was used to validate the result, and the Pearson correlation and recession analysis between the modified DRASTIC index and nitrate concentration depicted a strong relation with 0.76 and 0.7, respectively.

Vulnerability methods in hard rock formation as a basis for groundwater risk assessment – from resource to source

Groundwater in a hard rock formation is most endangered at places where a potential source can discharge contaminants that can reach the saturated zone of an aquifer. In these circumstances, an essential tool for groundwater protection is the contamination risk map. This map is based on the integration of two maps: a hazards map, i.e., map of potential sources of contamination and a vulnerability map. The selection of a proper vulnerability method is an important task since the resulting vulnerability map can significantly impact the final risk of contamination map. The most appropriate method for groundwater vulnerability assessment was considered in the case study of Tara National park, in Western Serbia. The four commonly used methods were applied to assess the intrinsic vulnerability maps: DRASTIC, EPIK, PI and COP. All the applied methods resulted in different vulnerability maps in assessing the degree of vulnerability, consequently influencing the groundwater contamination risk maps. The applied research presents an example of how contamination risk should be assessed in a specific area. Comparison of the results obtained for the area of Tara National Park indicates the preference of the PI method as a well-balanced method, taking into account all the specifics of the study area. A detailed analysis of the assessed risks in the catchments of the existing sources was also conducted to indicate probable sources of contamination and confirm the degree of accuracy of the created vulnerability and risk maps. The conducted research emphasizes the necessity to adopt a clear conceptual hydrogeological model and to apply several methods simultaneously to determine the optimal one for each individual area.

Assessment of a Multi-Layer Aquifer Vulnerability Using a Multi-Parameter Decision-Making Method in Mosha Plain, Iran

In recent decades, there has been a growing emphasis on assessing aquifer vulnerability. Given the availability of spatial data and the GIS advantages, mapping the groundwater vulnerability has become a common tool for protecting and managing groundwater resources. Here, we applied the GIS indexing and an overlay method to explore a combination of the potential contamination factors needed to assess groundwater vulnerability in the Mosha aquifer. The data from a borehole data logger and chemical analysis of spring water show groundwater responses to the surface contaminating sources. To assess the aquifer vulnerability, the potential contaminating sources were classified into three groups, namely (1) geological characteristics such as lithology and structural geology features; (2) the infrastructures induced by human activities such as roads, water wells, and pit latrines; and (3) land use. By considering these components, the risk maps were produced. Our findings indicate that the aquifer is very responsive to the anthropogenic contaminants that may leak into the aquifer from urbanized areas. Additionally, roads and pit latrines can significantly release pollutants into the environment that may eventually leak into the aquifer and contaminate the underlying groundwater resources.

Appraisal of groundwater pollution risk by combining the fuzzy AHP and DRASTIC method in the Burdur Saline Lake Basin, SW Turkey.

To ensure sustainable groundwater management, water resources must be protected in terms of quantity and quality. In this context, it is important to reveal the potential of existing groundwater resources to be affected by environmental and/or geogenic pollutants. In the present study, groundwater vulnerability assessment was performed using DRASTIC model by fuzzy AHP and GIS integration for Burdur Saline Lake basin, SW Turkey. In addition, validation and sensitivity analyses were performed in the study. Weight and rating values assigned for DRASTIC parameters and sub-parameters were determined by fuzzy AHP method, and these values were used in GIS analyses. In the validation analysis of the obtained groundwater vulnerability map, NO3 ion was considered since the agricultural activities are intense in the study area. In addition, sensitivity analyses were performed to determine the most effective parameter. According to the obtained results, 26.43% of the study area was determined as the highly vulnerable areas, while 11.44 and 62.13% were moderate vulnerable and low vulnerable, respectively. Impact of the vadose zone, depth to water table, and net recharge parameters were determined as more effective than other parameters. The nitrate concentrations of the groundwater in the region confirm the vulnerability map obtained by the study. Therefore, it is recommended to take realistic and urgent protection measures aimed at sustainable use of groundwater in highly vulnerable areas.

The assessment of groundwater vulnerability: A case study in the Doroud-Boroujerd aquifer, Iran

Background: Agricultural, industrial, and residential activities have caused the vulnerability of the groundwater of the Doroud-Boroujerd aquifer to pollution in Lorestan province, Iran. This study aimed to investigate the vulnerability of the Doroud-Boroujerd aquifer using a set of intrinsic (DRASTIC, IV) and specific (SI, LU-IV) vulnerability assessment methods. Methods: The DRASTIC model with seven parameters of groundwater depth, net recharge, aquifer media, soil media, slope, the effect of the vadose zone, and hydraulic conductivity of the aquifer has the highest number of parameters. The total dissolved solids (TDS) index was used to compare the efficiency of different methods. Results: The results showed two classes of medium and high vulnerability with an area of 73.71% and 26.3%, respectively, in the DRASTIC model. The SI model had two classes of low and medium vulnerability. The IV model had three classes of low to high vulnerability, of which the high class with an area of 75.94%, had the largest extent. The LU-IV model also included four classes of very low to very high (92.02%) vulnerability. The validation of DRASTIC, SI, IV, and LU-IV models with TDS index showed a weak correlation between vulnerability maps and TDS values, so it can be concluded that this index alone is not a good indicator for validation. Conclusion: The results of vulnerability assessment of different methods generally showed that the groundwater of this area is highly vulnerable, so it is recommended to take the necessary measures to prevent, control, and manage these valuable water resources.

Groundwater vulnerability assessment using DRASTIC model in Niwari District of Bundelkhand Region, Madhya Pradesh, India

Groundwater vulnerability assessment using DRASTIC model in Niwari District of Bundelkhand Region, Madhya Pradesh, India

A new approach to pollution vulnerability assessment in aquifers using K-means analysis

The most used methods to evaluate the vulnerability to contamination of aquifers are based on overlay index maps, such us DRASTIC, GOD and AVI. These methods assign weighting and rating values to hydrogeological characteristics, introducing subjectivity in the evaluation. In this research, a new methodology is proposed to eliminate some of that subjectivity. The methodology evaluates the vulnerability to contamination of a detrital aquifer using K-means cluster analysis with a new set of parameters. The set is composed of some parameters extracted from these methods, as well as other new ones that have a significant influence on the movement of contaminants. Application of the Principal Components Analysis (PCA) technique before using K-means cluster allowed the selection of the most relevant parameters. To validate the methodology, this was applied to a detrital aquifer located at central Spain (the so-called “Aluviales Jarama-Tajuña” aquifer) with a significant agricultural development. To compare the traditional methods of vulnerability assessment with the K-means cluster, nitrate concentration was used as a pollution indicator. Thus, 23 groundwater quality samples were used to correlate (Spearman´s correlation coefficient) the vulnerability values with nitrate concentration to validate the most suitable method. The results showed that GOD and AVI were not appropriate methods to evaluate the vulnerability of the aquifer, because they have negative or very low correlation with nitrate concentration (− 0.5 and 0.01, respectively). This is due to the use of very few variables that do not represent relevant features for the vulnerability assessment. Alternatively, DRASTIC and K-means cluster analysis obtained higher Spearman´s correlation coefficients (0.34 and 0.48, respectively). The relevant features selected by PCA analysis to use in the K-means low-dimensional analysis were depth of groundwater (D), net recharge (R), and land use (L). The new proposed method grouped data in three clusters that represent low vulnerability (35.9% of the study area), moderate (41.4%) and high vulnerability (22.7%). K-means increases the Spearman’s correlation by 14% with respect to the most approximate conventional method (DRASTIC). Therefore, the results obtained confirm the advantage of joint application of PCA and K-Means analysis, which represents a novel approach for the assessment of groundwater vulnerability in detrital aquifers.

Development of a vadose zone advanced monitoring system: Tools to assess groundwater vulnerability

AbstractPerforming repeat pore‐fluid sampling over long time‐scales can provide valuable information on unsaturated zone contaminants and their potential flux to ground water. This information can be used to manage groundwater remedies and identify contaminants that need to be sequestered in the vadose zone to minimize flux to ground water. Pore‐water samples are commonly used to obtain contaminant concentrations within the vadose zone, but existing methods are limited as they only provide a single sample at one location and time. The vadose zone advanced monitoring system (VZAMS) has been designed to integrate multiple technologies into a single down‐borehole system that allows for sampling of pore fluids (liquid and gas) to provide information about contamination and hydraulic conditions at multiple depths (∼0.3‐m intervals) within a cased borehole. Testing has been completed at the laboratory scale to verify the sampling elements of VZAMS, including geochemical testing for representative contaminants known to exist at the Hanford Site, located in southeastern Washington State. Physical tests focused on the ability of the sampler to draw fluid under unsaturated conditions. Initial geochemical testing showed that the stainless steel material used with the porous cuff may affect the sampled concentrations of redox‐sensitive contaminants under very dry conditions. Additional laboratory testing demonstrated that the VZAMS components are able to collect representative samples for substances of interest under expected field conditions. In this paper, the design and functionality of a novel instrument are demonstrated in support of subsequent testing in the field.

Hybridization of GALDIT method to assess actual and future coastal vulnerability to seawater intrusion.

In the recent years, the coastal aquifer of Jijel plain (North Algeria) located on the south of the Mediterranean Sea was utilized for cities growth and agricultural development of the region. Consequently, overexploitation and seawater intrusion were identified as major risks to the groundwater resource. In this work, a new approach integrating groundwater vulnerability method and numerical model for predicting the actual and future seawater is proposed. The groundwater vulnerability assessment has been performed by applying the GALDIT method using GIS and the MODFLOW model was used to simulate the actual and future groundwater level of the aquifer over the period 2020-2050. Three scenarios were simulated under water demand and climate conditions (drought, recharge) to obtain the changes in the groundwater level variation. The results of the GALDIT model application to the actual conditions (year 2020) showed that the high class of groundwater vulnerability is located in the coastal fringe and the terminal stretches of wadis where the seawater intrusion limit is located at a distance range between 840 and 1420m from the shoreline. However, the results for predicting future groundwater vulnerability showed that the scenario which proposed the artificial recharge basins, although predicting a worrying situation compared to the actual condition, has the best figure of the groundwater vulnerability assessment and seawater intrusion despite the other two scenarios. In this case the limit in the year 2050 is located between distances of 850-1640m from the shoreline with a forward speed of seawater intrusion of 1-8m/year, compared to the reference year 2020. This showed that groundwater level variation and recharge were the key factors in controlling groundwater vulnerability to seawater intrusion. The presented new approach can be used to mapping the actual and future groundwater vulnerability assessment to seawater intrusion and groundwater resources management in any coastal areas worldwide.

Groundwater vulnerability assessment in Randublatung groundwater basin using SINTACS-LU model

Randublatung groundwater basin is one of the crucial areas in Central Java that involves agricultural activities, which rely on groundwater sources. Agricultural activities have impacted the declining groundwater excellent of the Randublatung groundwater basin. The groundwater exploration was executed from an unconfined shallow aquifer that is in particular composed of alluvial deposits. Therefore, vulnerability evaluation to delineate regions most vulnerable to contamination from anthropogenic resources has come to be essential for sensible useful resource management and land use planning. This study estimates groundwater vulnerability by applying the SINTACS-LU model, which uses eight parameters to assess groundwater vulnerability and the geographical information system (GIS) in the Randublatung groundwater basin. The final subject level of SINTACS-LU (intrinsic vulnerability index) was created by multiplying the sum of each evaluation parameter by its specific weight. The resultant SINTACS-LU vulnerability map of the study area indicates that the areas most likely to be contaminated are between Kradenan and Keduntuban districts. Elsewhere, Rather high to high SINTACS-LU index values were observed, indicating areas of high vulnerability potential.

Assessment of the vulnerability of groundwater to anthropogenic contaminants in Kasihan, Bantul, Yogyakarta special province, Indonesia

Groundwater contamination is a ubiquitous problem, especially in developing countries such as Indonesia. Groundwater vulnerability assessment is an effective tool that provides information about the status of groundwater and potential contaminants. This approach helps in groundwater conservation and management. This study conducted an intrinsic groundwater vulnerability assessment to anthropogenic contamination in Kasihan, Yogyakarta, Indonesia. This research delineated the spatial distribution of physicochemical properties of groundwater and analysed land-use patterns. This study was conducted based on Groundwater confinement, Overlying materials and Depth to groundwater table (GOD) method. The GOD method was applied to assess the intrinsic properties of the geology and hydrogeology of the study area. Secondary land use data were analysed using GIS software to determine major anthropogenic activities in the research area. Land use and cover analysis of the study area indicated a rapid development including settlement, agricultural and industrial facilities. Groundwater vulnerability assessment showed medium to high classes. These findings suggest a high probability of contamination at present and in the future. Thus, future researchers should assess the groundwater quality of Kasihan based on specific vulnerability using groundwater flow and mass transport modelling because the investigated area is highly susceptible to pollutants.

Assessment of groundwater vulnerability using GIS-based COP model in the northern governorates of Jordan

Assessment of groundwater vulnerability using GIS-based COP model in the northern governorates of Jordan

Regional Scale Assessment of Shallow Groundwater Vulnerability to Contamination from Unconventional Hydrocarbon Extraction.

Concerns over unconventional oil and gas (UOG) development persist, especially in rural communities that rely on shallow groundwater for drinking and other domestic purposes. Given the continued expansion of the industry, regional (vs local scale) models are needed to characterize groundwater contamination risks faced by the increasing proportion of the population residing in areas that accommodate UOG extraction. In this paper, we evaluate groundwater vulnerability to contamination from surface spills and shallow subsurface leakage of UOG wells within a 104,000 km2 region in the Appalachian Basin, northeastern USA. We test a computationally efficient ensemble approach for simulating groundwater flow and contaminant transport processes to quantify vulnerability with high resolution. We also examine metamodels, or machine learning models trained to emulate physically based models, and investigate their spatial transferability. We identify predictors describing proximity to UOG, hydrology, and topography that are important for metamodels to make accurate vulnerability predictions outside their training regions. Using our approach, we estimate that 21,000-30,000 individuals in our study area are dependent on domestic water wells that are vulnerable to contamination from UOG activities. Our novel modeling framework could be used to guide groundwater monitoring, provide information for public health studies, and assess environmental justice issues.

Groundwater Contamination Risk Assessment Based on Groundwater Vulnerability and Pollution Loading: A Case Study of Typical Karst Areas in China

Groundwater contamination risk assessment is an important basis to support the protection of the groundwater ecological environment. In this research, the groundwater contamination risk in typical karst areas in China was evaluated by PLEIK model (P: protective cover; L: land use; E: epikarst development; I: infiltration conditions; K: karst development) and classification and discrimination methods were used to assess groundwater vulnerability and pollution source load; the water quality index method was used to assess the status of groundwater contamination in the research area. The results show that groundwater vulnerability values in the research area range from 3.04 to 7.79, and most areas have low groundwater vulnerability. Groundwater pollution loading indexes, in the region of the pollution sources, gathered numerical up-water quality status evaluation that shows that most of the regional groundwater quality in the research area has good groundwater at present. The groundwater contamination risk assessment results show that the groundwater pollution risk is mainly at a very low level in most areas of the study area, but the groundwater pollution risk is higher in the areas where the pollution sources gather. The result reveals that the regional groundwater contamination risk level for regional groundwater ecological environment protection provides a theoretical basis for policy making.