The rising point of the « Black Ernz »


The Luxembourg Sandstone layer, the most important ground water reserve in Luxembourg

Near the bor­der of the Grünewald, at the West­side of Ern­ster, the Black Ernz is ris­ing. After then it is cross­ing on a dis­tance of 20 km the north-east of Lux­em­burg, before flow­ing into the riv­er Sauer near the Grund­hof, a place locat­ed in the north-west of the town of Echter­nach. On its way it is cross­ing the famous land­scape of the Müller­tal, one of most beau­ti­ful region in Luxembourg.


Geo­log­i­cal section

Begin­ning of the rising

The springs of the Black Ernz are ris­ing out from the Lux­em­bourg sand­stone plateau, which is most often cov­ered with loose sand and woods, so that it is very spongy and rains can quick­ly enter the ground.


The exten­sion of the Lux­em­bourg sand­stone layer

The rain­wa­ter is first enter­ing the wood’s humus soil, which is play­ing a very pro­tec­tive role here by guar­an­tee­ing the water’s fil­tra­tion. Then the waters are cross­ing the spongy sand­stone, with a thick­ness up to near­ly 42 m.
This Lux­em­bourg sand­stone is a per­fect water car­ri­er all because of its numer­ous cracks and porosity.

At the meet­ing point of this sand­stone lay­er and the psilo­ceras planor­bis lay­er (stony and non per­me­able lay­er) the water can no longer con­tin­ue its way down and pre­cise­ly on this point the springs are ris­ing out.

The annu­al rain rate in Lux­em­bourg is about 2 bil­lion cubic metres from which 55% are evap­o­rat­ing, 41% flow­ing on the sur­face and only 4% can enter the soil to sup­ply the ground water.


The Black Enrz as an ele­ment of the hydrography

Most of these 4% are enter­ing the sand­stone lay­er, spread­ing over 493 square km but from which only 300 square km are spongy enough to let the waters enter. The oth­er 193 square km are cov­ered by stony and chalky lay­ers and can not play the fil­ter­ing role.
The Lux­em­bourg sand­stone is sup­ply­ing Lux­em­bourg with up to 90% of drink­ing water emerg­ing from ground waters (in Lux­em­bourg 2 thirds of all the drink­ing water is com­ing from ground waters and one third from sur­face waters).
The for­est ground, rich in humus is fil­ter­ing the rain waters.