פּירוש אויף בראשית 26:15

סתמום פלשתים THE PHILISTINES STOPPED THEM, saying, “They may become a source of danger to us because of marauding troops that may invade us” (i.e.,they may invade our country to obtain possession of the wells, or if they invade us for any other reason they will be able to find a supply of water). (Tosafot Sotah 10.) The rendering of the Targum טמונין means “stopping up” and we find it in the language of the Talmud (Pesachim 42a) “Chokes up (מטמטם) the heart”.

סתמום פלשתים, the Philistines had stopped up these wells to ensure that Avraham’s descendants would not be able to use them and to claim the land around them. as their grazing land. The Torah had to record this for us since Avraham’s shepherds had in the meantime pitched their tents a distance from there. We have a parallel to this when the Torah writes in 9,18 חם אבי כנען, Cham, the son of Canaan, although at the time, Canaan had not been born yet. Here too, the verse serves as background to the facts in verse 18, explaining that the servants of Yitzchok were not annexing land for their use which had not already belonged to Avraham with the full consent of Avimelech the king at time.

וכל הבארות, after Avraham had died the local populace was no longer afraid of Yitzchok. Also, the Avimelech with whom Avraham had made the pact and who had restored the wells to Avraham had died in the interval. Why then did the Philistines stop up the wells instead of using them for their own flocks? They were afraid that in the event that when Yitzchok would grow up he would be as powerful a figure as his father he would appropriate these wells. Therefore, they reasoned the only way to deny him these wells is by also denying them to themselves. Therefore they closed them up so that no one supposedly would be able to locate them.