Hydrogeological Evaluation of Fractured Volcanic Aquifer in Al-Sahoul Basin, IBB City, Yemen

The hydrogeological evaluation of fractured volcanic aquifer systems reveals the raw, unforgiving beauty and hidden water struggles of Yemen’s highlands. Nestled near Ibb City, the Al-Sahoul Basin exemplifies this: a fractured volcanic aquifer serving as both a lifeline and a complex puzzle for local communities. Drawing from recent studies and fieldwork, this analysis unpacks the basin’s hydrogeology, from its rocky foundations to looming groundwater threats. In a nation gripped by water scarcity, conflict, and climate shifts, understanding this aquifer is vital for survival.

Geological and Tectonic Setting

The Al-Sahoul Basin sits in Yemen’s western highlands, part of the Ibb Governorate, where ancient forces have sculpted a landscape of sharp ridges and fertile valleys. Geologically, it’s dominated by Tertiary volcanic rocks—think layers of basalt, trachyte, and ignimbrite from eruptions millions of years ago, overlaid on Precambrian basement of gneiss and granite intrusions. These volcanics, fractured by tectonic stresses from the Red Sea rift, form the backbone of the aquifer system. The basin itself spans about 428 square kilometers, a sub-basin of the larger Ibb watershed, with elevations climbing from 1,200 meters in the wadi floor to over 3,000 meters on surrounding plateaus.

Rainfall here is erratic, averaging 550 to 2,600 millimeters annually, but most evaporates or runs off steep slopes, leaving precious little to infiltrate the cracks. Tectonic faults crisscross the area, channeling water along lineaments that act like natural pipes in the rock. It’s this fracturing—born of volcanic cooling and seismic shakes—that turns impermeable lava flows into a viable water store, though unevenly distributed. Early surveys in the 2000s highlighted how these structures dictate flow paths, with some zones yielding bountiful springs while others stay bone-dry.

What strikes me is the interplay with overlying Quaternary alluvials: loose sands and gravels in the wadi beds that act as shallow reservoirs, recharging the deeper volcanics during flash floods. But overuse has strained this balance, turning what was once a resilient system into one teetering on depletion.

Hydrogeological Evaluation of Fractured Volcanic Aquifer Properties

At its core, the Al-Sahoul’s fractured volcanic aquifer is semi-confined, with groundwater moving through interconnected cracks rather than porous matrix—a classic hard-rock setup common in Yemen’s highlands. Saturated thickness varies from 200 to 250 meters, but productivity hinges on fracture density: high in fault zones, dismal elsewhere. Transmissivity ranges wildly, from 1 to 50 meters squared per day, reflecting the heterogeneity—storativity is low, around 0.01 to 0.001, meaning it doesn’t rebound quickly after pumping.

Recharge comes mainly from episodic rains, estimated at 32 millimeters per year in Al-Sahoul, feeding about 26 million cubic meters annually across the sub-basin. Springs and seeps along fault lines are key indicators, with discharge rates up to 10 liters per second in productive spots. Pumping tests from local wells reveal yields of 5 to 20 cubic meters per hour, sufficient for irrigation but vulnerable to overexploitation.

The water table sits 50 to 150 meters below surface, sloping gently toward the wadi, but recent declines—up to 3.5 meters yearly—signal stress. It’s a system shaped by lithology: basalts hold water best in their vesicular tops, while denser flows seal it in. This variability demands site-specific drilling, a lesson hard-learned in Ibb’s patchwork geology.

Methods of Hydrogeological Evaluation of Fractured Volcanic Aquifer

Evaluating this aquifer starts with a multi-tool approach, blending old-school fieldwork with modern tech to pierce the volcanic veil. Geophysical surveys like vertical electrical sounding (VES) map fracture zones, revealing resistivity contrasts that pinpoint water-bearing cracks—resistivities drop to 20-50 ohm-meters in saturated fractures versus 200+ in dry rock. In Al-Sahoul, over 100 VES points have delineated promising drilling targets, correlating low-resistivity layers with high yields.

Hydrochemical sampling follows, analyzing major ions (Ca, Mg, SO4, HCO3) and traces like nitrate and fluoride from 50+ wells. Piper diagrams classify water as calcium-magnesium bicarbonate types, fresh but edging toward mineralization in deeper flows. GIS and remote sensing amp this up: Landsat imagery extracts lineaments and drainage densities, while weighted overlays score potential zones based on slope, lithology, and rainfall.

Groundwater modeling via MODFLOW simulates flow, incorporating fracture networks to predict drawdowns—calibrated against piezometer data, it flags overpumping hotspots. Validation ties back to well logs and isotopes (like deuterium), tracing recharge paths from highlands to basin lows. It’s layered detective work, turning scattered data into actionable maps.

Groundwater Quality and Contamination Risks

Water quality in Al-Sahoul’s aquifer is generally potable, with pH 6.5-7.5 and TDS under 500 mg/L in shallow fractures, but threats lurk. Nitrate levels spike to 50-100 mg/L near farmlands, from qat fertilizers leaching into cracks—exceeding WHO limits and risking methemoglobinemia in kids. Fluoride hovers at 1-2 mg/L, beneficial in traces but problematic for dental health in prolonged exposure.

Heavy metals like iron and zinc creep in from agrochemicals, while sewage from Ibb City’s fringes contaminates alluvial interfaces, pushing E. coli counts over 10 CFU/100mL in untreated zones. GIS overlays reveal hotspots: 20% of the basin shows moderate pollution from landfills and septic systems, amplified by karst-like fractures that funnel contaminants fast.

Natural geogenic risks add layers—volcanic rocks leach strontium and copper, but levels stay below thresholds. Seasonal monsoons dilute issues, yet drying trends concentrate them. Monitoring networks, sparse as they are, underscore the need for treated wastewater reuse to safeguard this vital resource. If you want to know Earth science topics , click on it.

Groundwater Potentiality and Vulnerability Mapping

Mapping potential in Al-Sahoul reveals a mosaic: high zones (30% of area) cluster along major faults with dense lineaments and low slopes, ideal for recharge—think wadi edges where drainage converges. Moderate zones (50%) blanket gentler volcanics, while low-potential dry highlands (20%) resist infiltration due to steepness and basalt caps.

Using remote sensing, studies weighted factors like lineament density (40% influence) and geomorphology (20%), validating against 40 productive wells—80% hit high zones. Vulnerability flips this: DRASTIC indices score 140-180 for most, moderate risk, but urban fringes near Ibb tip to high (200+) from thin soils and high recharge.

Contamination models integrate land use, flagging 15% as critical—sewage plants and farms overlap fracture networks, accelerating pollutant spread. These maps aren’t static; climate models project 20% recharge drop by 2040, shrinking high-potential areas. It’s a call to prioritize: drill smart, protect fractures.

Challenges of Depletion and Sustainable Management

Depletion haunts Al-Sahoul, with abstractions hitting 32 million cubic meters yearly against 26 million recharge—a 23% deficit fueling 3.5-meter drawdowns. Qat farms guzzle 80%, their deep roots and thirsty habits drying wells faster than rains refill them. Conflict disrupts maintenance, while IDP influxes swell domestic pulls by 15%.

Projections grim: without curbs, exhaustion by 2029, deficits ballooning to 42 million cubic meters by 2040. Solutions start small—rehab 11 spate dams to harvest 16 million cubic meters, channeling floods to fracture inlets. Drip irrigation could slash ag use 40%, freeing water for recharge pits that boost infiltration 25%.

Longer-term, regulate drilling (cap at 200 meters) and crop swaps—millet over qat. Wastewater from Ibb’s plant, expanded to 20,000 cubic meters daily, recycles 7 million cubic meters for fields, cutting pollution. Community co-ops monitor levels via apps, tying aid to conservation. It’s tough in Yemen’s turmoil, but phased plans—short-term fixes by 2027—offer hope.

Conclusion: Charting a Wetter Future

Peering into Al-Sahoul’s fractured depths, I’m reminded how geology and human needs entwine—volcanic cracks that once quenched highland thirst now strain under modern demands. This evaluation spotlights a resilient yet fragile aquifer: productive where fractures align, vulnerable where pollution prowls. With GIS maps guiding drills and policies curbing waste, we can tilt toward sustainability.

Yemen’s waters demand global eyes—aid for dams, tech for monitoring. For Ibb’s farmers and families, it’s personal: every meter saved is a harvest secured. Let’s learn from these rocks, forging paths that honor the land’s ancient flows.